MDK is available for free on GOG.com!

I highly recommend to get this version, which IMO is way better than the one on Steam (less problems, if any!). Also, it has some cool goodies like wallpaper, game manual in form of Dr.Hawkins' journal (crazy stuff) and the original soundtrack made by none other than Tommy Tallarico.

Only 44 hours left so hurry up and tell your friends.