Qt
I've been using Qt for a number of years to develop GUI applications on Linux. And have recently completed a number of applications which I've cross compiled to run on Windows, Linux and OSX. I have also recently been working with the Series 60 port of Qt for Symbian, which has opened up a whole lot of new opportunities.


Mono
Mono can be used to run both GUI and console based applications on a Linux system. This is often useful if you have ASP.NET web services but no Windows machines to run them on. Mono has proven to be extremely stable, and has run almost all the applications I've thrown at it without any modifications at all, which I'm fairly impressed with.