Gallium is a modern graphic stack were you have a single driver that supports multiple graphics APIs. Instead of having to have separate drivers for OpenGL vs X Render.... you just have application libs that tie into Gallium 'State Trackers'.
* Mesa OpenGL state tracker
* DirectX state tracker
* OpenVG state tracker
* X state tracker
* OpenCL state tracker
The state tackers keep track of what application is requesting what and the Gallium driver combines them into GPU instruction primitives that end up getting piped to the graphics card. Want to support a new GPU-level API? then make a new tracker. The trackers should be mostly hardware agnostic so they can work just as uniformly well completely regardless of what your using for acceleration... it could be a video card driver, LLVM, or some sort of weird future GPGPU. Write once, compile everywhere type thing.
There really is no such thing as 'OpenGL acceleration' on video cards anymore. Those are long dead. Like before ATI R200 dead type dead. Instead what we have is all-software-stacks that are compiled to run on both CPU and GPU, using whatever gives the best performance or is the most available. It's complicated, but it works and GPU is now a native part of the PC architecture and can be taken advantage of much like a math coprocessor from 486DX days was used. The GPU is just another processor.
That way instead of forcing everybody to write new APIs by abstracting on top of OpenGL or whatever else is lower, the application developers can just use whatever is best for their purposes and the driver takes care of it for them. This is the way things should be. This is something that Wayland is going to need to have underneath it in order to be better.
Currently working Gallium drivers exist for Nvidia and AMD/ATI video hardware, but it's all a HEAVY work in progress. Only very recently have they gotten OpenGL working on real hardware.