Supreme Snowboarding is a pretty nice snowboard simulation. It contains all the modes you would expect, including a championship, time trial, free races and stunt modes. It has a multiplayer mode you can play on a LAN. A lot of cool stuff is possible as a lot of stuff is written in configfiles, readible by any text editor. If you like you can change the board speeds, designs, character descriptions, or thebehavior of sow or rock. More information can be found here. This site also maintains the world records.