In the beginning of April 2010, when I answered this question, I researched Alamo, Avis, Budget, Dollar, Enterprise, Hertz, National and Thrifty and checked out their
tabs or links for Car Rental Deals and/or Car Rental Specials. (One other thing that I suggest is that you sign up for email alerts from various companies... they usually offer this option on their website, often when you check the car rental deals or specials offers).
While Alamo didn't appear to have any one way car rental specials between Phoenix and Denver for drop off, they were offering:
Arizona one way specials
between Phoenix, Mesa, Glendale and Scottsdal - dropping off to Anaheim CA, Orange County CA, Cleveland OH, Burbank CA, Sacramento CA, Eugene OR, Fresno CA, San Francisco CA, Medford OR, Long Beach CA, San Jose CA, Portland OR, Los Angeles CA, Santa Barbara CA, Redmond OR, Monterey CA, Kansas City MO, Oakland CA and Las Vegas, NV
Avis was offering the following deal:
Drive out of any participating Arizona location (Phoenix was one if them) and get money-saving rates, unlimited mileage and no drop-off fee. Simply return the vehicle to a participating location in Arizona, California or Nevada. There weren't any participating drop off locations in Denver.
This one way car rental special was found under the DEALS tab onAvis.com
-- Just click on the Avis logo on the upper left of your screen.
Budget was offering the following one way specials from Phoenix
Pick up your Budget vehicle at any participating Arizona location (Phoenix was one of them) and drop it off at a different, participating location drop off location in Nevada, Arizona or California. There were not any one way specials between Phoenix and Denver as a drop off.
At the time it was available, this special was found under the SPECIALS tab onBudget.com
National was also offering similar Arizona one way specials; but again there was no participating drop off locations for Phoenix, only select locations in California, Missouri, Nevada or Oregan.
This offer was found on the CAR RENTAL SPECIALS TO GO link on nationalcar.com
Dollar, Enterprise, Hertz and Thrifty were not offering any one way specials at that time.