The major players I'm aware of are Java, QT, GTK. I'd probably favour QT over GTK, I get the impression it's more mature for crossplatform work. I also think Skype uses QT, and it actually works well on both Linux and Windows, unlike most Java apps I have used. Am I missing any obvious choices? Are any of these toolkits actually horrible to work with?
The app will be fairly basic, it just needs to do some network comms and graphics display.
Thanks for the help :)