I use quite a few addons, but the following ones have helped me with my achievements:
Ackis Recipe List
WarcraftPets - my pet collection
Warcraft Mounts - my mount collection
El's Extreme Anglin'
WoW Tabards - my collection so far
Two sites provide an immeasurable amount of help:
And finally, for world events and supermassive achievements, I owe a lot to The OverAchiever column on WoW Insider, which gave me the confidence to go after those meta achievements that just look daunting on-screen!