sleddog, MicroSizers are new, but they are really the US name for Bit Char-Gs which have been out for some time. You can buy cars and hop-ups from the Shop:
http://tinyrc.com/shop/products.php?pid=2
The bodies come with spoilers, but you can often find other cars (e.g. Hot Wheels) as donators for new spoilers etc. Have a look around the Forums, you'll find people are doing all kinds of things to these cars!