If you like any sports at all, ScoreMobile. It's free through BlackBerry App World. Has every sport you can think of, easy to use, scores, stats, betting lines, schedules, recaps, set teams as favorites, and also has live blogging on some games.
That, and Facebook is always nice to have on your BB.
I would say two of the most useful besides the obvious (facebook/msn etc etc) are "anagram" where you can highlight senders details in an email and automatically add it to your address book and "forward,reply & Edit" so you can do just that, which for some reason you cant already on your bb.... both very useful!
Another good one is Slacker Radio, also free in the BlackBerry App World.
There's a free version where it only lets you skip 6 songs per station. Think Pandora.
I would suggest Last.fm, but it does'nt give the option to DL. Plus I've read that it has A LOT of bugs.
Posted from my CrackBerry at wapforums.crackberry.com