Hertz is offering a deal that waives the typical $25 underage fee for 21-24 year old renters through the end of September, 2011. You just need the Promo Code 144314 when making the reservation. More details on their website.
Can't think of another time I've seen that on a large scale basis, but certainly it could make for a good deal for anyone under 25 looking to do a roadtrip this summer!