i would advice on using qt designer,
http://qt.nokia.com/
its really cool and lets you build apps on linux and windows

not to mention when you get really good at coding you can also make apps for nokia ! and get paid for your apps cool right and its free
as for MS .net or c# C++ they are nice free tools but they can not simply run on linux without some big complie changes anyway i think if your really into coding you should go with qt designer and head towards C++
good luck !
--------------------- updated post ! sorry to say but it looks really bad if your looking to charge fees with qt
OK I'm really not sure what to say after I've read the following from QT
http://qt.nokia.com/products/licensing/ ... al-license
so in order to develop apps which you want to charge users you need to get Qt Commercial Developer License, so i will take back my words and tell you QT
is not what you want if you are going to start charging users fees for your application
if you develop open source apps qt is really great as videolan.org is built using qt
I'm eating my words now, QT Commercial Developer License is expensive i would rather jump out of a bridge then pay that fee not to mention the qt run-time fees , there are probably some other frameworks you can build around with C++ and be able to mess on linux and windows
geez i hate coding