I bought the Kensington car mount from Amazon after reading a review on xda. The mount is made for an iphone but works perfectly with the Evo (no case). The mount is only $23, holds the phone tight, swivels from portrait to landscape and comes with both a windshield and vent mount. I have only used the vent mount and can tell you it works very well. You might also want to check buy.com as a few weeks ago they were selling this mount for $10 shipped after rebate. For the price it can't be beat.
YouTube link -
YouTube - Kensington iPhone Car Windshield/Vent Mount, HTC EVO 4G Approved!
Amazon -
Amazon.com: Kensington Windshield /Vent Car Mount with Quick Release Cradle for iPhone/iPod Touch: Electronics