Kensington Dash/Friction Mount for IPhone
Accessories for mounting iPhones, GPS units, and the like in your car come in two flavors: those that attach to your windshield--such as Just Mobile's Xtand Go ( Macworld rated 4 out of 5 mice )--and those that sit on your dashboard. (Minnesota is the only state in which it's illegal to mount such a device on your windshield; California lifted the restriction in 2009.) Of the dashboard options, there are adhesive types that require you to place something sticky on your dash in order to secure the mount, or friction mounts such as Kensington's Dash/Friction Mount with Sound Amplified Cradle for iPhone.
A friction mount uses both weight and a non-slip bottom to let you plop the mount anywhere there's room on the dashboard and have it stay put (as well as to let you easily move the mount between cars). The Kensington model weighs a hair less than two pounds, and in my testing, it was incredibly stable--at least in the first car I tried. In another car, I couldn't find a large enough flat area on the dashboard, and the dashboard was too close to the windshield, for the mount to be of any use. It simply wouldn't fit.
(The mount actually comprises three pieces: the weighted base, the cradle for your iPhone, and the adjustable, rotating arm in between. The arm connects to the base using a suction cup, which means you could instead mount your iPhone to your windshield using just the arm and cradle--a nice bit of flexibility, considering Kensington sells the arm/cradle combination alone for the exact same price as the entire Dash/Friction Mount package.)
Kensington's big selling point for its mount is that it amplifies the audio coming out of your iPhone's built-in speaker by directing the sound through a horn-shaped space in the cradle. Kensington claims this design can double the iPhone's output volume, which would be especially useful when you're listening to the turn-by-turn driving directions from an iPhone GPS navigation app to reach your destination safely. I couldn't measure the volume improvement scientifically, but my iPhone's audio output was noticeably louder with the Kensington Mount than with other car mounts I've tried.
To get the maximum volume, Kensington includes four sets of rubber inserts to help form a tighter seal with your particular iPhone model. There's even a set for using the mount with an iPhone in a case, although the company warns that with so many case designs on the market, the sound-amplification design might not work properly with every one. (The Mount accommodates encased iPhones well--my iPhone 3Gs with a Marware case fit just fine.) Unfortunately, the inserts are small and frustrating to change. And that's not helped by the fact that Kensington doesn't include any instructions in the box, instead sending you to an online, silent video that's not particularly educational.
Macworld's buying advice
If you have room on your dash for a car mount, the Dash/Friction Mount with Sound Amplified Cradle for iPhone will do a good job securely holding your iPhone while keeping its screen in plain sight. And its sound-amplification prowess makes it easier to hear whether you should turn left or right at the next corner.