wxWindows
 

Cross-Platform GUI library

An open source C++ GUI framework to make cross-platform programming child's play. Well, almost.

What is wxWindows?
wxWindows 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, wxWindows 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.

http://www.wxwindows.org/

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Lars