Oh, absolutely. Just messing with the visuals or stats of games was an artform in and on itself.
Speaking of which... we used to have a world-class "4D Driving/Stunts" tournament that ran from 2002 to 2007 (it was actually my first time seeing Europeans in-game!) and one constant point of argument was the inclusion of user-made cars.
Amazingly, the site is STILL UP:
http://wrcstunts.free.fr/
And so is my friend's Sims site (last updated: 2004, before I even met her):
http://chasque.net/dana/
This site has survived everything that it links to, including its very own guestbook!