2D/3D Scene Graph rendering engine for Linux/Unix/Win32 layered atop OpenGL. Supports stereoscopy, volume rendering, 2D/3D texture mapping, picking, LOD and view-dependent operations, cross-platform text, constant-rate rendering and image-based data.
FOX is an Extensible C++ Class Library for Multi-Platform GUI development. Platforms supported include Linux, Windows NT/9x/2K, and UNIX/X11 workstations (IRIX, Solaris, HP-UX, Tru64, AIX, FreeBSD, and others).
Clawlib is or intends to be a cross platform C++ library
SOAP module for Apache. Supports C++ server side components (classes) as well as other languages.