One of my favorite little freeware racing games, Gene Rally finally got an update after almost 7 years last month. You can get it at
http://gene-rally.com/.
While there is a default Monster in the game, I made some privateer MJ trucks back in college for it, and they are available in the forums at
http://forum.gene-rally.com/viewtopic.php?f=22&t=933 (you will probably need to register). There is also a Chicago-Style track I made (that actually works!) at:
http://forum.gene-rally.com/viewtopic.php?f=21&t=934
Car editor is available at
http://www.generally.com.ar/
If you've never tried GR before, it's a fantastic little throwback to Super Off-Road and the like. The game is only a few megs, and there are literally thousands of cars and tracks available for it - browse the forums and find some real gems.