Coverage is a superb, single-purpose app for the iPad and iPhone. Its single purpose, as the name implies, is to compare carrier coverage in the USA. It shows you 4G and LTE coverage and more for each of the four major US carriers (AT&T, Verizon, Sprint, and T-Mobile).
Here's a little more background, via the App Store page for it:
The only app that directly overlays and compares carrier coverage maps! Perfect for travelers – research where you’re covered across the USA!
Coverage provides an at-a-glance, “universal” cellular data coverage map by interactively overlaying maps from major US cellular providers.
It focuses on helping mobile bandwidth junkies know where carriers claim they might get their next hit.
** Know before you go: Coverage has standardized each major carrier’s data map so they can be easily overlayed, creating a personalized coverage map of major carriers you travel with.
** No internet needed: Our proprietary coverage maps are stored locally; check where coverage is most likely even when you have none. (The underlaying maps require internet/caching.)
** Urban and rural areas: Maps cover the entire continental USA, Alaska, Hawaii, Puerto Rico, and the US Virgin Islands at a regional level.
** Carriers included at higher-res: AT&T, Sprint, Verizon & lower res: T-Mobile.
** Know where to roam: view LTE, ’4G’ (HSPA+ & WiMax), 3G, 2G and roaming areas as inclusive overlays, focus on finding the coverage type you need. (Hint: If your device indicates ’4G’ – it may be HSPA+/WiMax not just LTE. For AT&T and Sprint customers, use the “4G” setting to see both.)
Read the rest of Coverage App : Compare 4G and LTE Coverage Maps on the iPad and iPhone (289 words)