There aren't great cross-platform tools that provide good native-feeling results, mostly. There are a number of cross-platform toolkits. GTK+, Qt and wxWindow

There aren't great cross-platform tools that provide good native-feeling results, mostly. There are a number of cross-platform toolkits. GTK+, Qt and wxWindows all come to mind as possibilities. But in general, you're going to need to rewrite a lot of the front end as native code if you want it to feel like native code. However, if you know you may be porting, you can separate a lot of the back end out of your UI code and make it a separate library in separate files. That makes it much, much easier to write several different front ends and still share most of your code.