Useful ones I made up on the long flights about Azeroth.

/use [button:1] Roasted Quail;[button:2] Morning Glory Dew;[button:3] Tender Wolf Steak
(if you shiftclick the item, it prints the name in the macro box for you)
Click it with left mousebutton = Eat
right mousebutton = Drink
Middle mousebutton(or scroll wheel click) = "Enhanced food" (STA/SPI boost)

/cast [button:1] Humanoid tracking;[button:2] Beast Tracking;[button:3] Hidden tracking
For hunters to bind the 3 most important tracking to one button :) again, LMB=humanoid, RMB=beast MMB=Hidden (rogues/druids)

/castsequence reset=120 Blood fury, rapid fire
Killing Spree!!! Hit it once for blood fury (if its CD is ready), second time (or if CD wasnt ready) Rapid fire :)

/cast [modifier:Alt] Immolation Trap;freezing trap
Drops a freezing trap unless you're holding Alt (this is perfect for binding to a key I.e [F])

/castsequence Strength of earth totem,searing totem
Low lvl enhance shaman, two presses, same button gives 2 totems.

:) Binding multiple skills (tracking/aspects) to a macro has helped clear my UI alot!

/equip Blue brewfest hat
/equip Tuxedo Jacket
/equip Tuxedo pants
/equip tuxedo shirt
/equip yellow brewfest stein
/equip ritual stein
Puts on your suit, hat and gets out your tankards :D
If there was more then 255 chars allowed Id add in:
/use brewfest pony keg

Hopefully I haven't spelt any wrong. Not near the game atm to check. :)