I have been writing up a systemless adventure module and in it I came up with the following table filled with a bunch of strange effects that could be either good or bad for some dungeon crawlers. Here is a link for the PDF.
D100 Strange Random Effects Table
Saving Throws are suggested for these strange effects but that is left to the Game Master to implement. Also the effects below are for only one random character in the area and not the whole group of characters. Finally it is left up to the GM if these effects are permanent or if they are only a temporary effect. "You" and "Your" in the descriptions refer to the character affected by these effects. If a strange effect gives a character a spell per day and they normally can't cast spells this effect surpasses the normal restrictions to magic use.
1) Two random Stats increase.
2) You develop a persistent hunger for the flesh of your own race. A Saving Throw could be used to resist this urge when it becomes too strong to resist.
3) You do not need sleep for 2D4 days.
4) Lucky you! Roll twice against this chart and apply both effects.
5) One Skill increases a minor amount, or increase a Stat.
6) One random hand and one foot shrink to half size.
7) One Skill increases a medium amount, or increase a Stat.
8) Your legs turn into a mass of useless tentacles.
9) One Skill increases a major amount, or increase a Stat.
10) Your face melts off leaving a skeleton visage. Social interactions suffer a major penalty, but you get a bonus to intimidation attempts.
11) Hit Points go up a minor amount.
12) Your skin liquifies and as it sloughs off it cries like an infant.
13) Hit Points go up a medium amount.
14) Your hair turns into red beetles and they scatter in fear.
15) Your mind is transferred into a random item that you are carrying. You can control your body as long as it holds the item your mind is within.
16) Your teeth fall out after crumbling into ashes.
17) When you deal melee combat damage you deal twice as much.
18) Your mouth seals shut via a thick membrane of living flesh.
19) You double in height.
20) Your hands are replaced with constantly dripping mounds of putrescent goo.
21) Learn one additional language.
22) Somewhere in the world (or in this dungeon level) an evil person has their appearance changed to resemble yours.
23) Your skin changes at will.
24) You will start worshiping the next thing you see (player's choice) as a god.
25) You take half as much damage from any wound.
26) Drinking water causes you to turn into a goat for 1d4 minutes.
27) Only magical weapons can harm you.
28) Your body hair now glows brightly in the presence of evil, giving off enough light to illuminate a fifteen foot radius area centered on you.
29) You no longer feel pain, but you are less likely to notice that you are injured and thus less likely to seek aid for your wounds.
30) You must now vomit your stomach acid onto your food before eating it (like a fly). This can also be used as an acid based attack within a short range.
31) Only blunt weapons can harm you.
32) Hit Points go down a minor amount.
33) Only slashing weapons can harm you.
34) Animals hate you, even once dearly beloved companions. You automatically critically fail any husbandry or riding related rolls.
35) Only piercing weapons can harm you.
36) You are convinced that you died an especially gruesome and dissatisfying death and that you now live your unlife as a ghost that haunts the rest of your party. No amounts of evidence can convince you otherwise. Once a week you can make a Saving Throw to know the truth.
37) You gain 1 spell that can be cast once per day.
38) You can't speak for 1D6 days.
39) You gain 2 spells that can be cast once per day.
40) Your hair grows 1D10 yards.
41) you grow a pair of fully functioning wings. You can fly with your normal speed but you can only wear custom made armor.
42) Your skin blisters as if it was burned.
43) You are 1D6 years younger.
44) You must say everything sarcastically.
45) A minor deity, angel, or demigod shows up and offers you one minor boon. It will refuse tasks that it cannot or will not perform.
46) You are 1D6 years older.
47) You're a lefty now! Inverse main hand. For several weeks you will suffer from a lack of coordination but after this period of time you will become Ambidextrous.
48) Your intestines turn into snakes that bite the inside of your body. There is a 1 in 6 chance that they are poisonous.
49) You gain 1 spell that can be cast twice per day.
50) A character's airways are closed 2/3rds of the time.
51) You gain 1 spell that can be cast three times per day.
52) You cough out bloody feathers every few minutes.
53) You can breathe in water and in air.
54) You cough out writhing worms every few minutes.
55) Your skin provides a minor amount of damage reduction.
56) Dungeon walls speak to you, but they have nothing good to say. They berate you and antagonize you, and conspire to alert monsters of your whereabouts. They loudly reveal your tactics to any within hearing distance. Attacking the walls begets mockery. 57) Your skin provides a medium amount of damage reduction.
58) 1D10 screaming worms crawl out of the flesh of your arms dealing damage in the process.
59) You develop two fiendish and fanged mouths on your palms. They snarl and try to chew anything you touch. Any living thing that you touch is harmed and you are healed an equal amount. If you haven't fed them during the day they will try to self-cannibalize you at night. You are more intimidating now than you were before.
60) Your eyes become unusable pools of blood.
61) You can levitate at will.
62) You take twice as much damage from any wound.
63) Your fingers and knuckles become moving stone. Damage done with your fists is tripled.
64) Children just don't like you now. They cry constantly and vomit in your presence.
65) You vomit out 1D20 silver coins every 10 minutes.
66) Your hands become feet and your feet become hands.
67) You vomit out 1D20 gold coins every 10 minutes.
68) Your eyes turn into valuable gemstones but you can still see out of them.
69) You understand the languages of animals.
70) Whenever you speak your true name all light sources are extinguished within a twenty feet radius of you.
71) You understand the languages of birds.
72) When you stare at any fire you see a face of a grinning demon or devil in it.
73) When you hear music you hear subliminal messages that contain helpful tips and truths.
74) Your insides are replaced with insects and spiders. They have a hive mind but your mind is now just one voice in a chorus of other voices. They are able to wear and manipulate your body as they see fit. Physical Stats go down a medium amount, and mental Stats go up a medium amount. You take extra damage from all attacks that aren't spells or psychic in origin.
75) When you look at a book it speaks the words that you are staring at.
76) Your toe and finger nails grow an inch every minute.
77) Learn 1D2 languages.
78) All your stats go down a minor amount. You are convinced that you feel stronger, more intelligent... etc.
79) Learn 1D4 languages.
80) You forget 1 language for 1D6 days.
81) Your hair is constantly blown back by a magical wind (possible Stat increase).
82) Every time you look in a mirror you will see a monstrous version of you staring back. If you sleep near a mirror the monster will creep out of it and try to kill you and replace you. If you kill the monster while it is out of the mirror the curse will be broken.
83) An ancient intelligent artifact of bygone days materializes before you and will help you to achieve your goals.
84) You leave putrid black footsteps everywhere you walk.
85) Every liquid you drink turns to wine as it goes down your throat. If you drink a potion it has the same effects it just tastes MUCH better.
86) Your shadow gains a will of its own. It has an opposite alignment or temperament.
87) Water turns to blood when you touch it with your left hand.
88) You think that there is a loud talking bird on your shoulder but it isn't actually there.
89) Your insides and blood are searing hot. You gain a high bonus to resist diseases, parasites, and poison because of it. When melee weapons wound you there is a moderate chance that your spilled blood could burn your attacker.
90) When you speak or write, others cannot understand it. They perceive it as an unknown language.
91) A small horde of roughly 20 vermin begin to follow you around and if allowed to do so will protect you to the best of their ability, but they will only act to defend you.
92) You see your heart leave your body as it bursts out of your chest. The wound instantly seals up. The heart however is still beating (somehow pumping blood through your body) and you are still alive. You must now protect your heart while it exists outside your body or you will die.
93) You fade out of existence when you sleep into a safe pocket dimension. When you return upon waking you are fully refreshed.
94) You break out head-to-toe in unsightly, painful, and leaking pustules. People respond negatively when they can see your face and any social interactions suffer penalties for it.
95) You produce a shadow of light when in darkness that has the same brightness of a dim torch. You have no normal shadow when in sunlight.
96) You are convinced that you are becoming some sort of supernatural or extraplanar creature. You refuse all evidence to the contrary.
97) All nonmagical mechanical locks automatically unlock when you draw within ten feet of them.
98) You can no longer rest or sleep. After 2 days you suffer penalties to all activities. Every day after that you may make another saving throw. Death may result from this effect, or if the GM wishes they are simply in a constant state of exhaustion. 99) Gravity affects you less. You get a bonus to swimming rolls as you tend to float naturally, and whenever you fall you will float down slowly.
100) Your two arms merge into one centered on your chest. Any Stat or Stats that represent combat, strength, or athleticism goes up a medium amount but now you only have one arm.