Each language is rolling out its own GC. Why not share some efforts in a common library?
As mentioned above, I'm regularly talking with Ravenbrook about liberalizing the license on MPS and doing some work on modernizing it. I've hopes that that might happen this year.
And some of the language specific ones are pretty good ... SGen in Mono seems interesting. The GC in ClozureCL is interesting as well.
libgarbagecollector is a plug-in garbage collector library designed to be used by high level language implementors. It's used in the Io programming language.