SnowfallKeyPress : WoWInterface Downloads : Action Bar Mods
Makes it so all spells are used on keypress, instead of on keyrelease in some cases.
For example: Press and hold Frostbolt. The frostbolt doesn't go off until you release your keybind. With the addon, the Frostbolt will start casting on keypress.
Miss fewer interrupts, decrease the milliseconds between each cast, increase your DPS if you're a PvE nerd.