Some will be covered, but my favourite apps so far;
Facebook - pretty self explanatory
FlickR - pretty self explanatory
Last.fm - pretty self explanatory
Twitterific - chose this over Tweetie, as it was free, might get Tweetie if i decide to use Twitter more..
IM+ Lite - can't make myself pay the extra for no adverts, and they don't bother me too much, good app
Sky Mobile TV - absolutely awesome, really surprised by the quality of it
Sky+ Record - pretty handy, I always forget to record stuff, so it helps
Best Camera - my newest app, anyone into Photography should get it, really nifty and kind of similar to the FlickR community
Air Mouse - pretty nifty, use this to just control my Mac when i'm not near it
Remote - easier to control music as i can see my library, and play music anywhere in the house
Allowance - prefer this over MyO2
Flick Fishing - really addictive, even though its just fishing
Fling - really addictive time waster
Tap Tap Revenge 2 - as above, play this pretty often
Flight Control - another good time waster
RedLaser - don't use it that often, but its pretty cool, have saved a few quid using it, actually!
AroundMe - very handy, can find shops/restaurants etc quickly
Shazam - i always hear a song i don't know what it is, this has found quite a few of them so far
Got a few more, but mostly just ones i downloaded for the sake of it, above are pretty much my favourite ones.