I like the Kenu Airframe smartphone car mount ($20) so much that I gave the one I had to my father so he could use it on a road trip. I ordered a replacement.
I like it much more than suction-cup mounts, which don't stay stuck. The Kenu has a clip that attaches to your car's heating/AC vent, and spring-loaded expandable jaws that grip your phone securely (for phablets use the
Kenu phablet mount). When I travel, I bring it along to use in rental cars.
Cheapskates might want to try the
iMeshbean Air Vent Car Mount, which costs $5. I ordered one to find out if it's any good.
Note: If you have hot air blasting from the vent, it's probably not a great idea to have your phone in front of it. I live in Los Angeles so I rarely use my car heater.