i already know java, c++ (no one framework for GUI development) and little of c# (winforms for gui development) but i'm thinking to ivest my free time to learn Qt, what do you recommend me? has Qt a good future or not?
I think it's worth it. We used Qt recently because it allowed us to make a cross-platform GUI that looks good on Linux, Windows, and Mac. It's kind of a tough learning curve to figure out how to use the QLayouts correctly, but after you do your GUI will look great on many different platforms, with many different font sizes, and with many different window sizes. I like how Qt allows you to make applications that use the native widgets with the native look and feel. Applications built with Qt will look like they belong no matter what platform they are running on. Another great feature is that the Qt framework is small enough that you can just include the Qt DLL files in your application's binary directory, so it simplifies the installation process for your application and makes life easier for everyone (no VMs to install, no 600 MB downloads).
You can make cross-platform GUIs with C# (using mono on Linux and Mac), but IMHO the results in Linux look terrible.