But there's so many!

planb - essential for Black Orcs, auto-rotate action bars based on plan state
keeptracker - see status of keeps across all tiers/realms
ISAssistTracker - get your group organised
sendstuff/junkdump/junkdelete/bulkmailer - sort your stuff!
fastfriends - sync friends across alts
objectivetracker - see timers on BOs in your tier pairing
targetinfowindows - see enemy class/type
scrollminder - never fly without a guild recall scroll again

And many more.

issit