Lazy_pig:
-hold shift to autosell gray items and autorepair
-autodismount
-stuff like auto-accept BG queue, group invites by guildies or friends, chat filters for different stuff
-right click to put items into trade, mail or AH tab to sell, alt+right click to search for items on the AH
-very good GUI with many options (/LP)
CT_mailmod:
-mass mails!
-get all mails or selective mails without having to delete them 1-by-1
Informant:
-item value (sell to NPCs, very useful while grinding and not knowing the worth of stuff while having no room in bags)
-tooltip in which professions an item is being used
WIM(if you whisper a lot (raid/guild leader or organizing stuff), this is a GREAT addon):
-instant messenger type whispers with many many configs(/WIM) for when and how to pop-up extra whisper windows
-whisper history for each character who has whispered you (up to a 2 wees iirc)