Make your own ability!

[TheHuntBegins]

You know the drill.

Adrenaline

Raises Attack, Sp. Attack, and Speed in a pinch for three turns. No longer usable after switching out.

Retreat

Negates blocking moves & abilities. (Shadow Tag, Arena Trap, Block, Mean Look, Fire Spin, etc.)

Point-Blank

Fights up close and doubles Sp. Attack. (like Pure Power & Huge Power)

Panic

Raises Attack when confused.

[Apprentice]

Hmm.......

Amythest

Raises the power of Ice-Type moves when HP is less than 1/3. (Like Overgrow, Blaze, and Torrent)

Threshold

Sharply raises Attack when hit by a Rock-Type, Fighting-Type, or Steel-Type Physical move.

[Liam]

No letback

A Pokémon's attack and special attack is increased when not holding an item.

Meh, I will think of more soon. :p

[Apprentice]

I've got more!

Target

Super-Effective moves against your opponent get a higher critical hit ratio.

[TheHuntBegins]

Take Turns

All Pokémon on the field move in a different order each turn.

In single battles, the faster Pokémon moves first the first turn, then the slower Pokémon moves first next turn, etc.

In double battles, goes 1-2-3-4, then 2-3-4-1, etc.

[The Macintosh Ninja: SOH CAH TOA]

End of Days
All Pokémon on the field will faint after 5 turns.

[MonsterMon64]

Echo Voice
Voice and sound moves' accuracy increase by 1/2.

This would be very helpful on Pokémon like Jigglypuff/Wigglytuff, because (if I did my math right) this would raise Sing's accuracy to 82.5%. It's not perfect, but it's a significant enough change to make Sing remotely useful. More accurate than a standard Hypnosis, at any rate.

[__Strange__]

Sympathy

In double battles, damage taken by any Pokemon is cut in half - one half to the attacked Pokemon, one half to it's partner.

[TheHuntBegins]

Distraction

In double battles, moves that hit more than one Pokémon now only hit Pokémon with this ability. Target is random if more than one Pokémon with this ability is on the field.

Instinct

Automatically uses a super-effective move when given the chance.

Pull Through

Attack does not drop when burned, nor does Speed drop when paralyzed.

[Bakura]

Sweet Aroma

Grass Types Of The Opposite Gender Will Not Attack This Pokémon.

Leaf Shield

Raises Def and Sp. Def In Strong Sunlight. (50%)

Dark Power

Doubles Attack. Attack Is Halved If Pokémon Gets Low On Health. (Half and Below)

[Mik]

Friendly Fire

In double battles, if a move hits your team mate, it does double the damage. (Earthquake, Surf, etc)

[q.c.™]

Dragon Fury

Powers Up dragon moves when HP is low.

[Enigma]

Sparks

This Pokemon's Speed increases when it is hit by a Steel type attack.

^Only certain steel types would have this (Steel hits steel = sparks?) and so it wouldn't be all that broken.

Photosensitive

This Pokemon receives damage when Sunny Day is in effect, but the accuracy of its moves are increased by 50%.

Chameleon

This Pokemon gains STAB on moves that share the same type as the attack it was last hit by - e.g. if Keckleon is hit by a Fire type attack, its Fire type attacks will do much damage.

[TheHuntBegins]

Revitalize

Attack and Sp. Attack increase when cured of a status.

Power Short

Immune to Electric-type moves.

[Coppo]

Pre-emptive Strike

When the foe uses an attack that's super effective, this Pokemon moves before it.