May I chip in:

Seesmic, for Twitter

Voice Note, to send yourself a voice memo by email

Songkick, reads your MP3s, creates an artist list, tells you when they’re in town

HulloMail, if you are charged per message for voice message retrieval by your carrier (might only work in UK)

FolderSync, automagically syncs to a variety of cloud stores

Sipdroid, best of the SIP phones [I haven't tried the JB stock one yet so it may have improved]

Elixir Widgets, control and toggle widget manager, has a companion app called Elixir 2 for all sorts of phone info and stats

WifiMatic, watches your location by cellphone towers, and turns your wifi on and off accordingly. Excellent battery saver.

Moxier, the least bad connector to Exchange/Outlook, from a crowded field of bad choices. This is the only one I could find that would successfully sync Notes and Tasks.

Market Enabler, allows you to come into the market as if you were from a different country

Gesture Search, quickest way to look up contacts, with an added warm glow of Palm Pilot nostalgia