Note: HammerCast no longer works as of WoW 2.0.
HammerCast is great for casters. It allows you to hammer your spellcast keys without the game giving you annoying, clacking or vocal feedback ("Clack! Clack!" like the sound of a clipboard snapping shut)--or worse--interrupting your channeled spellcasts. This way you get your spells off as quickly as possible.
Basically all it does it ignore actions while you're casting or channeling.
I haven't officially released HammerCast, but email me if you're interested.