ClanLib is a cross platform toolkit library with a primary focus on game creation. The library is Open Source and free for commercial use, under the ClanLib License.
Feature Highlights

Very liberal zlib style license
Supports Windows, Linux and partially Mac OS X
OpenGL, Direct3D and Software Rendering
High-performant shader based render architecture
Full GUI framework customizable using CSS 2.1
GUI windows rendered to textures or native windows
High level 2D graphics with fonts, sprites, animations, collisions
Network library with sockets, web and game interfaces
Game IDE graphic interface
Physics for both 2D and 3D
Scene Manager for 3D
OpenCL support
Database supporting SQLite
Sound supporting wav, tracker formats and ogg-vorbis
Basic cross platform runtime (threading, file I/O, XML/DOM, etc.)
Template based signal library (type-safe callbacks)
Integrated resource management