wxWidgets gives you a single, easy-to-use API for writing GUI applications on multiple platforms. Link with the appropriate library for your platform (Windows/Unix/Mac, others coming shortly) and compiler (almost any popular C++ compiler), and your application will adopt the look and feel appropriate to that platform. On top of great GUI functionality, wxWidgets gives you: online help, network programming, streams, clipboard and drag and drop, multithreading, image loading and saving in a variety of popular formats, database support, HTML viewing and printing, and much much more.

"Excellent, stable and intuitive API. Very straightforward to learn and easy to port Java, X11 and Win32 code to."
"Thanks heaps for the best piece of software I've ever come across."
"wxWidgets 2 has been an absolute dream."
"I've never had an easier porting experience."
"I never thought that cross-platform development could be so easy and simply cool."
"I have used wxWidgets in the past very successfully on multiple projects, and think it's the bee's knees. Thanks for everything!"
"wxWidgets is jaw dropping amazing. Community support from the mailing list is extraordinary. Are you sure this is free?"
"wxWidgets is one of the most magnificent development projects in existence.."