On my BlackBerry, I tend to use MS Live Search (as much as I dislike Windows Mobile, this is actually really handy--still in beta); Opera Mini (also a beta version); Yahoo! IM (2.0--not AT&T-approved but there's a workaround), and Google Maps. Any of the map applications are more useful on a BB with GPS. PocketDay is also nice but I find myself using it less often lately.

These are just the third-party apps--I also use BlackBerry Messenger (included) all the time.