A wonderful instant messenger, now better than WebMessenger (NEW):
VeriChat for BlackBerry
IM+ Instant Message Software for BlackBerry

A relevant thread is:
Midlets: You can use Nokia/Motorola Java apps on Blackberry!

And also this thread:
RSSReader for BlackBerry (XML Newsfeed reader)

And yet another thread:
MicroSky astronomy viewer

An open source ssh utility (not as full featured as Idokorro MobileSSH):
midpssh