Stops the current spellcasting. Doesn't work for channeled spells.
Returns nil if it didn't do anything (no spell was being cast at the moment), 1 if it did stop a casting.
Can disable autorepeated "spells", like hunter's Auto shot.
A recent post from the WoW main boards suggests that you can link two or more spells/abilities to fire from the same macro press (and that this move is legal) as long as none of the abilities (except the last one) triggers the global cooldown.
This is done with the following simple setup (two spells in one macro).
/cast Spell Without Global Cooldown /script SpellStopCasting() /cast Spell With Global Cooldown
Or, for more than two spells.
/cast Spell Without Global Cooldown /script SpellStopCasting() /cast Spell Without Global Cooldown /script SpellStopCasting() ... /cast Spell With Global Cooldown
It's been tested to work with Nature's Swiftness (Shaman/Druid), Divine Favor (Paladin), Inner Fire + Focused Casting (Priest), but should work for any ability that does not have a global cooldown attached.