Hertz and Budget have had their programs for a couple of years now. Alamo had a good bare-bones program called True Blue, but they dumped it in about 5-6 years ago.

I haven't participated in Budget's program since 2001, but at the time it was the most rewarding in terms of dollars-spent-per-free-day. You could earn 1 free day per each $240 spent, whereas Hertz was 1 per $600. At some point in there Budget cut the points in half, I believe. You could also redeem your points for years-out-of-date golf equipment, which was kind of funny. The biggest problem, of course, is that Budget provides unreliable/unpredictable service at many of its locations. For that reason, I abandoned Budget (redeemed the points for a 1-iron that I probably should have bought off of eBay for $20) and switched to Hertz. Their program isn't quite as rewarding, but it is administered well and the free day certs show up quickly when I redeem the points.

Hertz also sends me another free day cert after every 15 rentals. I think. Maybe it's 10 or maybe it's 20 - I get two or three of them a year. True FREE day certs - no minimum keep and very few blackout rules.