To provide for easy testing of the Mesa GLSL compiler while being separate from the rest of Mesa driver implementations, Ian has re-separated the compiler into a standalone compiler branch. The Mesa compiler tends to try to conform to the upstream Khronos Group specifications as much as possible, so with making the compiler stand on its own it's trying to become somewhat of the reference implementation and to serve as a "GLSL Lint" implementation.
Aside from sticking as close as possible to the published specifications, the Mesa GLSL compiler is good for being able to build on many different operating systems / platforms and will hopefully allow third-parties to better determine whether there's a problem in their own GLSL shader or a driver behavior problem. The only downside, of course, is that Mesa's GLSL support is still a fair ways away from matching the latest upstream Khronos specification -- there's nearly the GLSL support to match OpenGL 3.3, but most of the GLSL changes for the GL 4.x specifications remains to be implemented.
For more details on this standalone compiler work to assist OpenGL game/application developers, read Ian's blog post.