I use quite a bunch of Android apps. Regularly, I stick with those though:
[*]Trillian (there's no better IM for Android out there although it's not free)
[*]LauncherPro (yep, the paid version)
[*]AndFTP (now anyone tell me there's ANY better FTP client?)
[*]Titanium backup (saved me quite a lot of work so far)
[*]Handcent SMS (flexible, configurable SMS app)
[*]SuperPower (keeps my battery away from drowning)
[*]Angry Birds (all 3 games :D)
[*]Seesmic (great Facebook AND Twitter client)
[*]Smooth Calendar (helps me keep track of what's going on)
[*]Swype (typing is soooo 90s)
[*]K-9 Mail[/list]
On random occasions, I also use:
[*]AndChat (the least ugly IRC client I could find)
[*]Voodoo Control Plus (better sound driver for Samsung phones)[/list]
edit:
Oh, and Firefox. How could I miss it? Might be slow but feels better than Opera stuff.