SuperTuxKart is a free and open source attempt at making a Mario Kart clone, and to be honest, it's pretty decent.
In fact, I recommend it for those who love FOSS games for the following reasons:
1. For starters, it's quite well put together. It's very stable, quite fleshed out, and has a lot of content out the gate.
2. It tries to do it's own thing instead of straight up ripping off Mario Kart. The physics engine has several differences with Mario Kart while still having similar gameplay, the characters are based off the logos of other FOSS software (like Wilbur, the logo animal of the GIMP program) and each has their own unique playstyle.
3. It's multiplatform, so Windows, Mac,and Linux users can all enjoy it.
About the only downsides is that it is pretty huge (500 MB for the Windows version) compared to Mario Kart (the Mario Kart 64 rom is around 8MB), but in this day and age that's pocket change in terms of space, and if you want a Mario Kart style fix for free, you could do a lot worse.
