In alphabetical order:
Battery Graph – useful when figuring out what eats battery
Dolphin HD – best web browser by a mile
iPlayer – you knows it
iNap – if you often fall asleep on the train
Quickpic – because the HTC gallery browser stupidly always starts at your oldest pictures
RealCalc – a proper calculator
Tasker – 110% of awesomeness
Twicca and Tweetcaster – the best Twitter clients IMO; former best for single account, latter for multiple
Viewranger – the best logging, navigating and tripcomputering app
Vignette – for setting up a page of shortcuts to useful settings, including fixed focus modes for fast shooting