This is probably the closest thing we'll get to a PC version of Mario Kart, er, ever.
Well, that was fast. Only a little more than a day after Valve added karts/bumper kart combat to Team Fortress 2, someone's already released a Mario Kart mod with three levels. It's mostly rooted in the Halloween event's brand of treat-centric trickery, so your item selection is for now limited to the stuff Valve's put out.
The mod uses a modified version of Lifeless Raceway, a map created for use in another pre-existing mod that modders then further modded to mod mods moddingly and — oh gosh, where'd I go just now? Sorry about that. Anyway, that's part of why mod maker Zool was able to get this one out so quickly.
Give it a go, if you're feeling so inclined. And thank every heaven that blue shells don't exist in this one. Yet.