Style is the ongoing C++ port of IBM's SWT for Java, itself deriving from IBM's VisualAge for SmallTalk, with two major twists. First, it is written in 100% ISO-C++ with no specific compiler dependencies, and second, it comes with a threading model design that guarantee that users won't be stuck with busy cursors forever. It currently targets OS X, Win32, and GTK.
ZWM is an SDL-based window manager and widget library intended to be used in cross-platform applications. It features basic widgets like buttons, lists, containers, a file browser, combo-box, and many more. It also features support for mutimedia playback (MPEG, MP3, WAV, MIDI, Ogg, etc.).