I'm getting better rates booking directly through Alamo than on Autoslash (sorry for the short notice, I'm headed out for a quick weekend to PHX).

My reservation:
Start Date: 19 Jun 2010 5:30 PM
End Date: 20 Jun 2010 5:30 PM (1 day weekend rental)
Location: PHX
Car Class: cheapest available
Memberships: AAA, Costco, AmEx, several other travel-related

Booking on alamo.com with Corp ID 7014627 (Costco - 10% off) with coupon AU2729MJW (1-class upgrade) nets me a Compact class car (upgraded to Midsize) for $10.41. After taxes and fees, a grand total of $21.58.
For an even better deal, apply Corp ID 7014667 (20% off) and coupon AU2852SDA (2-class upgrade). Use the same Compact car rate of $10.41, but end up with a Standard class car for a total of $20.24.

Even when I put in various combinations of corp ID's and coupons on Autoslash, the best I can get it down to is $12.93/day, $26.19 a/i. Autoslash also confirms me into a compact car, and won't let me start/end my reservation at 30 minutes past the hour.

I know the $6 difference isn't much, but I pay for all my own travel so every little bit helps.