Well, at least you can still download the free trial:

http://download.microsoft.com/download/monstertruck20/Trial/...

Listed as "Abandonware" in one of their linked Fan sites and readily downloadable if you can't get the trial to work.

http://vales.com/mtm2/

I doubt it's public domain, but I doubt Microsoft would go after you, too.

That's sort of the definition of abandonware - the publisher has apparently given up trying to make money off the thing but also not explicitly released it for free either.