This is a collection of tracks for Mario Kart Wii. Most of these models have been made into playable levels which can be played on on a Wii using an SD card. Quite a few of these are in CTGP Revolution, a very popular distribution of custom tracks that can be played online. I also maintain a collection of CT Components:
http://sketchup.google.com/3dwarehouse/cldetails?mid=12edfdd0b2ebcc3a2f0dce64c393d5a2
Visit this site to download levels like these and learn how to play them on your Wii: http://wiki.tockdom.com/index.php