VeriChat -- My favourite 3rd party app.
MobileSSH -- My other favourite. I've got a Linux box to access, as well as for relay service.
Berry411 -- Free 411 utility (though I'm considering upgrading to Handmark Express from
http://www.handmark.com ....)
PDTool - Utility to change the ReplyTo: in emails.
All listed in these two threads:
http://www.blackberryforums.com/viewtopic.php?t=336
http://www.blackberryforums.com/viewtopic.php?t=3178