The host and star of MTV's show called jackass. He is a pretty good skateboarder* as are the rest of the cast members. In fact, they often hang out with the likes of Tony Hawk, who was on one of the shows. Basically, this guy is completely crazy. He does all kinds of insane stunts like getting shot with a taser, jousting on BMX bikes, being set on fire, and many others. He is the basically the spokesperson for don't try this at home.
Ok, Some new information has come to my attention...
MTV's jackass is based on/created from video shot mostly by/of Bam Margera and his friends for a series of skateboarding videos including CKY, CKY2K, and CKY3. These videos are very similar to the show, except that there is more swearing, more bare female breasts, and a little more actual skateboarding. They were originally created to showcase the skateboarding talents of Bam and his friends, and the musical talent of the band Camp Kill Yourself in which Bam's brother is a member. It seems as though they have been fairly successful, since Bam drives a very nice Audi A4 in one of the earlier videos, and an even nicer Audi S4 in one of the later ones.
*Compared to me, that is. Compared to the rest of the cast, Knoxville really isn't a very good skateboarder. Thanks to 409 for kindly pointing out my error!