Fucking hippies and their "I don't need a car, why do you?" bullshit.
Christ. Can't ask a simple question without getting greened on.
The two cheapest of the national chains are Enterprise and Dollar. The Dollar on Boren has been good for me.
But to go real cheap the very best thing to do is either put an ad on Craig's list looking for what you need or...
Buy a piece of crap. There are plenty of Chevy Celebrities etc out there and some go for as cheap as a hundred bucks- don't pay more than $500. Registration is about $60- taxes are dependent on the price you tell the licensing agent you paid (low ball them).
Insure the thing at geico or progressive or something (usually less than $50 a month or so if you have a decent driving record).
Drive the car. Afterwards, sell the thing for near what you paid, cancel your insurance. Conceivable, you just had a car for month for about $150 or so.