wxWidgets Is a mature GUI toolkit that uses each platform's native API, so provides a native look and feel from one codebase. This makes it more thoroughly cross-platform than some other libraries.
It has bindings to many languages (more than are listed on the homepage - check your language's package libraries for a "wx" prefix).
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wxWidgets was started in 1992 by Julian Smart.
Pro Truly native look and feel
It uses the platform's native API rather than emulating the GUI.
Pro Open source
Pro Can do custom control rendering
You can make a custom renderer using the class wxDelegateRendererNative to draw directly any standard control or using the class BackgroundWindow to use any bitmap as a control background.
Con Uses GTK on X11
WxWidgets uses GTK+ on X11 desktop which well known for looking like an alien on non GTK+ desktops.