XTN5250 - a super simple and fast Java 5250 Emulator
Not much else to say. The only real problem I can see with this 5250 Emulator is it defaulting to PORT 23, and I cannot see a way to change it. This might affect you if you are on a site where this needs to be over-ridden.
Other than that - XTN5250 simply works!
Grab the current version from SourceForge or grab my mirror copy from here