Popular Content:

http://www.hotwheels.com/
The official Hot Wheels site.

http://www.hotwheelscollectors.com/
"The" site for serious Hot Wheels collectors.

http://en.wikipedia.org/wiki/Hot_Wheels
Wikipedia's Hot Wheels page.

http://pages.ebay.com/buy/guides/hot-wheels-buying-guide/
eBay's guide for buying Hot Wheels.

Hot Wheels Are Worth How Much???
An interesting article about the value of Hot Wheels today.

http://www.hallsguide.com/
Hall's guide to Hot Wheels.  A generally entertaining site for Hot Wheels collectors.

http://www.southtexasdiecast.com/hwguide.html
An excellent guide to nearly all hot wheels.  If you haven't seen this, you should.