Bukkit

Airstrike: Calls in a TNT airstrike on target block (does not do block damage)
Arrowblast: Arrowstorm built-in without the reagent cost
Arrowstorm: shoots a random number of arrows at a random rate of fire
Basher: passive chance to stun the opponent when you attack
Blaaze: summons 1-3 wild blazes at target block
BurningPrescense: toggle-able passive that deals aoe damage
Chaser: deals damage to the target + extra damage if they don't move
Chicken: summon 1-3 chickens at target block
Cow: summons 1-3 cows at target block
Creeper: summons 1-3 creepers at target block
CreepingPain: Takes all damage dealt to the target during the duration and deals it to the target upon expiring
Cripple: deals damage to the target + extra damage if they move during the duration
Critical: passive chance to do extra damage when you attack
Dampen: passive prevents enemies from hurting you if they have below a certain amount of mana
DeathFromAbove: You deal your fall damage to opponents near you (safefall optional)
Despair: AOE blindness + damage
Detect: Detects a block type within range at eye level in a straight line
Disguise: (/skill disguise zombie) Disguises you as a mob for a duration or until you use it again.
Dodge: passive chance to dodge enemy attacks
Drain: passive drain mana from your opponent when you attack
DropTNT: Drops armed TNT at your feet. (Does not do block damage)
Empathy: deal damage to opponent = health you're missing * multiplier
Escape: blink + invuln + dispel on self (can be used while stunned)
Equalize: heals you or your target until your hp equals theirs.
FalconPunch: Stun, but can also optionally do an explosion that wont hurt players and can optionally destroy blocks
Firespin: light target block and all blocks around it on fire
Firewall: create a wall of fire at target block
FullHeal: heals you to full hp over time. Cancels if you take damage.
Glassshield: create a moving shield of glass and air
Grace: passive gives you pvp immunity for a duration after you die
GuardianAngel: gives you and nearby party members Invuln
Healbomb: heal yourself or an ally and deal area of effect damage around that person
Hunt: Teleport to a random block X blocks away from your target (/skill hunt playername)
IceAura: toggle-able passive that deals damage to all players around you
Icebrand: passive chance to slow enemies as you attack them
Inversion: deal damage to the target = amount of mana they are missing * multiplier
InvertDamage: turns all incoming damage into heals for the duration
Jail: curses an enemy for a duration. If they die during that duration, they respawn in a jail (location set in config)
Lifesteal: passive gain health when you attack a person (not creature)
MagicArmor: passively reduces damage from magic sources. (most skills are magic)
ManaPotion: gives you mana
Megabolt: instantly strike lightning on everything around you
Mercy: toggle-able passive that prevents pvp between players too high or too low of a level difference from you (level range is a multiplier of your level)
MortalWound: prevents the target from being healed for the duration
Mooshroom: summons 1-3 mooshrooms at target block
Multibolt: strike target block with lightning, in there was a person there, chance to strike that player again, and again, and again
Multiply: spawn 1-3 more of target creature
OmegaGrip: Channeling stun that drains mana and health
Pandemic: area of effect damage over time, but cannot kill a player (will leave them at 1hp)
Pig: spawn 1-3 pigs at target block
Reagent: does nothing on it's own
Reborn: passive that gives you half hp if you were about to die (has a cooldown)
RegenMana: passive multiplier for mana regen
Sheep: spawns 1-3 sheep at target block
Skele: spawns 1-3 skeletons at target block
Slow: Slow + damage
SonicBoom: Area of Effect damage + silence
Spider: spawns 1-3 spiders at target block
Spines: passive chance to do Barrage when hit
Stun: stuns + damage to target
Tackle: charge + stun + damage
Tether: root dispel + pulls everyone around you to you if they move too far away (can be used while stunned, but won't dispel the stun :O)
Timebomb: creates a pit of 1-4 TNT at target block (works best if tnt damage is enabled but block damage is not) the pit erases itself after the duration
Track: (/skill track playername) reports the target's world, location, health, class, level, and whether you can hurt them or not as well as reports your world and location.
Tree: spawns a small tree at target block that lasts for 45 seconds
Unheal: causes any heals on the target during the duration to be converted into damage
UnholyRitual: UnholyRitual except you can set the amount of mana it gives you
Unmark: Removes any marks near you (requires SkillMark which is a private skill)
Vitalize: Periodic heal + mana for the duration
Void: Gives you pvp immunity and invuln against creatures
VoidBlade: passive chance to silence enemies as you attack them
Woof: Summons and tames a wolf for you. (might just summon it though, not sure)
Wrestle: Stuns everyone around you
Zombie: spawns 1-3 zombies at target block