"Can you hear me now?" It's more than just a TV commercial—we've all been victim to spotty coverage, loss of signal, and the dreaded "my cell phone works at work but not at home" syndrome. Is it just me, or do other people have the same problems where I do?
DeadCellZones.com maintains a wireless coverage complaint database. You can check out their map to see who's having problems where. Color coded by carrier, it may help you to see that you're not alone, or to decide which new carrier you don't want to sign up with. You can report your own problems as well, and maybe help somebody else avoid troubles in the future.
In addition to the map, they've got useful information about coverage issues (beware the big concrete and steel office building), and links to individual carriers' coverage maps.