It's the stock air filter from a Duratrax Firehammer. For the money, it's the best thing out there IMO. Dual stage foam filter with a metal shell. Price? A wallet shattering $16.99!!!

http://www2.towerhobbies.com/cgi-bin...p?&I=LXHTX0&P=

I run my car in EVERYTHING, and as you can see from some of the pics above, it gets VERY dirty. Air filter is almost always spotless everytime I check!