The Memory Pool System or MPS is a GC that was originally written back in the 1990s at Harlequin where it was used in their DylanWorks environment. When Harlequin went under, some members of the team bought up the rights to MPS and now manage its development at RavenBrook.
MPS is interesting as it allows for a number of collection strategies and supports precise and conservative collection modes.
It has excellent documentation: http://www.ravenbrook.com/project/mps/master/manual/html/
A number of links in the original post were broken, so see Richard’s follow up for some correct links: http://mailman.ravenbrook.com/pipermail/mps-discussion/2014-March/000134.html
DylanWorks became Open Dylan where we still use MPS on some platforms. We use Boehm on others, but are working on a compile-time switch that will let us swap between Boehm and MPS on all platforms.