Sparkfun already offers the fun and imagination-firing [Tamiya tank treads and [dual-motor gearbox (as well as a bunch of compatible wheels). Unfortunately, the tank treads are not really useable without some sort of chassis to mount them on :).
Tamiya makes an excellent general chassis solution, the [Tamiya Universal Plate. It would be nice if these were available right here on the site (to avoid shipping charges from multiple sources. What can I say? I’m cheap!).](http://www.superdroidrobots.com/shop/item.asp?itemid=465)](http://www.sparkfun.com/commerce/product_info.php?products_id=319)](http://www.sparkfun.com/commerce/product_info.php?products_id=321)