A few favourites from my (rooted) SGS2:

Epistle - a simple text editor which transparently syncs to Dropbox.

YATA! - my favorite timer app

Chordbot - really cool for trying out music ideas

d7 reader - No nonsense google reader client

UPnPlay - for playing audio/video off my NAS

MortPlayer Books - audiobook player

QuickPic - best gallery I've found

Pinboard - for saving bookmarks

...and games: GameBoid, psx4droid, Snes9x, UAE4Droid (all emulators), ScummVM, Wesnoth, Nethack, Xenowar...