Gwydion Dylan is an implementation of the Dylan programming language for Unix systems. Originally written as a research project by the Gwydion group at CMU, it is now maintained by a group of volunteers.
The current version of Gwydion is development code, and intended only for use by Dylan fanatics. The compiler is slow, shared libraries are somewhat fragile, and overall still needs lots of bug fixes. To make life more exciting, the documentation is incomplete, and you'll need to read the source and ask questions on the mailing list. If this sounds like fun, you'll enjoy Gwydion.
Thanks to the skilled programmers at CMU, Gwydion can already generate exceptionally efficient code (about the speed of C in most cases) and implements about 98% of the Dylan standard with many extra libraries.