Welcome to Android!
Where to begin, in no particular order:
1) Tweetdeck. if you're a twitter fanboy, Plume works well, too, especially for widgets.
2) Angry birds, of course. Orginal, Rio, Seasons.
3) Opera Mobile 11 or Dolphin HD for a replacement browser. Try both to see which you prefer.
4) Amazon Appstore for the free daily(usually piad) app--good stuff on there daily.. ie. fruit ninja, angry birds rio, swift key
4)Chrome-to-phone. you'll need an extension on your desktop to use this, but it's really nice when you want to send a link or file to your phone right from your desktop/laptop browser.
5)Pulse. VERY nice RSS feed. PLENTY of sites to choose from + you can add your own if it isn't included.
6)xda-app/tapatalk. Makes forum reading very easy
Many, many more.. Remember that you can browse apps from the comfort of your desktop browser by visiting market.android.com.
I'm not sure what the situation will be for custom roms, but, if possible, be sure to root and get voodoo sound installed. Night/day difference!
Good luck!