While Cairo (GTK+ uses Cairo) and Qt are most often talked about when it comes to graphics libraries on Linux, Skia is actually more common than most people probably realize even though it's talked about less. Skia is used by Google's Chrome/Chromium web-browser, Chrome OS, and Android. In fact, Google acquired the company developing Skia more than a half-decade ago.
Skia provides an API for drawing text, geometries, and images. This library includes support for 3x3 matrices, anti-aliasing, transparency, filters, shaders, transfer modes, mask filters, and path effects. It can be looked at as being most similar to Cairo, rather than Qt or other tool-kits, as this is less of a library with native widgets but rather Skia just natively handles drawing various primitives.
Skia is made available by Google under the Apache 2.0 license and, of course, is multi-platform. Platforms currently supported include Linux, Android, Windows, and Mac OS X. There's also planned ports in the future to WinMo, Symbian, and Apple iOS.
One of the items that does make Skia interesting is its support for various front-ends, which allows taking SVG, PostScript / PDF, Flash, and Illustrator formats as input for then translating into Skia API calls.
Skia's drawing has back-ends for a standard CPU-based software rasterizer, PDF, and OpenGL. There's also partial back-ends for OpenGL ES, OpenVG, SVG, and Adobe SWF/Flash. The OpenGL back-end is what will interest most for utilizing GPU hardware acceleration. All API calls can be accelerated on the GPU via OpenGL as they are passed through the SkCanvas, of which there is SkGLCanvas for translating everything to direct OpenGL. It's somewhat like Clutter.
The back-end support is similar to that of Cairo, but that LGPL/MPL-licensed library also has experimental back-ends for DRM (direct to the Linux kernel Direct Rendering Manager drivers) and also as a Cairo Gallium3D state tracker. Cairo also has an X Render back-end if all you have is an X.Org driver providing 2D RENDER acceleration.
Unfortunately as Skia, Cairo, and other 2D graphics libraries don't share any common API or front-end, it's not trivial to directly compare the performance of these libraries and their respective back-ends for OpenGL, etc. If anyone though has any other test requests, feel free to let us know.
More information on Skia can be found on the Google Code project site.