My personal favs are pretty much standard awesome apps:
Google Sky (just for the wow factor. I found Mars last night!)
Shazam (figures out what a music track is for you)
Torrent-fu (great if you download torrents - you can manage the torrents and start new ones on your PC from your phone)
Quick Settings (one-click access to varios settings such as brightness, ringer, bluetooth, mobile data. gps, airplane etc)
NetCounter (keep tabs and set up alarms for data transfer limits either through mobile net or on wifi)
Advanced Task Killer (don't use it much at all, but an essential app if you manage to download a rogue program that has an unruly process!)
ASTRO (navigate your phone with folders/files, search for specific files, def essential!)
Barcode Scanner (can scan various barcodes, especially good for scanning app barcodes found on some websites)
Speed Test (in case you're ever curious what speed transfers you're getting)
Various time-killing games
Tangram (lite or pro, great for train journeys)
Trap (trap little balls between lines you create. sounds rubbish, is actually awesome)
Mystique (horror type find-items-and-combine-them-to-escape-the-room type game)
Fun Towers (pyramid solitaire. force closes on load occasionally, nothing major)
Furdiburb (a lil virtual alien pet type thing. Still in beta and there's not much to do, but has the potential to be cute)
Gem Miner (I'm not sure if I'm taken with this one, but I have the free version)
Pocoro (logic-type move the ball game)