While Alamo doesn't appear to have any one way specials between Phoenix and Denver for drop off, they are 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

You can find this under the HOT DEALS tab at Alamo.com

Avis is currently 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.

You can find this one way special under the DEALS tab on Avis.com -- Just click on the Avis logo on the upper left of your screen.

Budget is 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. You can find this under the SPECIALS tab on Budget.com

National is 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.

You can find this offer on the CAR RENTAL SPECIALS TO GO link on NationalCar.com