Those super-cheap "hi grip" tires on eBay
Just mounted a set of "6081" and ran them around on the street for a bit.
Verdict: Not bad
Pros: they are very lightweight, (super-thin) and kinda stretchy. Seem to handle on asphalt surprisingly well (comparable to HPI X's, ish)
Cons: they have no structure whatsoever and the foams they come with are hopeless. Balloon like crazy, too. The tread is very shallow over a very thin carcass so I have some doubts as to how long they'll last.
Make sure to build them with tape on the inside (anti-ballooning) and some beefier foams.