Mario Kart: Source
I really do love the mod community, especially when Half-Life has one of the best by far, in fact, I don't really pay attention to any other mod communities other than those that are developing for Source. The latest is Mario Kart, still in very deep development from what I can tell but still very cool. They have one of their karts nearly done as shown to the right and it looks fairly cool. They have some conceptual work for characters and a few images of maps. Did I mention they have the block world from the n64 Mario Kart? Awesome. Though I have yet to play any of the automobile related mods from source I intend on giving this one a run through once it is an alpha stage.
Check out Mario Kart: Sources Website.