Sure when you go to a carrier's website you can see a map of their coverage and it usually looks pretty awesome. What about in the real world? DeadCellZones catalogs user-reported dead zones to show you real-world coverage.

Plug in a zipcode, city, or street address into DeadCellZones and it scours the 100,000+ entry database of user-submitted dead zones. You can filter the results by viewing all carriers, individual major carriers like AT&T, Sprint, T-Mobile, and Verizon, or you can—less helpfully—see a generic "other" category for smaller carriers.

You can zoom in and click on individual dead zones and check for additional information like whether or not the zone in inside, outside, and other details. DeadCellZones is a free service and requires no login or registration to use or submit dead zone reports.

For another 3rd-party opinion look at previously reviewed GotReception to check coverage in your area. Have a favorite resource for digging up the dirt on cellphone companies, coverage, and equipment? Let's hear about it in the comments.