Usability 
I spent my vacation last week roaming around the US Midwest including some time along the shores of Lake Michigan in Northeastern Wisconsin. It's a beautiful area and the Northern latitude and breeze off the lake make it a lot more temperate than sweltering Chicago and Milwaukee. But it's also an area were the nationwide GSM operators, ATT and T-Mobile, don't have any towers. My ATT GoPhone prepaid SIM was useless there but T-Mobile To Go, also prepaid, offers extensive off-network roaming at no extra charge. I was happily talking and texting on a couple of local operators, Einstein PCS and iWireless.
Being a curious techie I wanted to find out more about these operators. Some Goggling eventually landed me on gsmworld.mobi, the mobile web version of GSMWorld.com, which is the official site of the GSM Association a global trade association of GSM operators. GSMWorld.mobi has coverage maps and a bit of other information about over 700 GSM operators worldwide.
This site could be especially useful when traveling internationally. Use it to find carriers with coverage in the places you'll will be visiting so you can purchase the right pre-paid SIMs.
The mobile coverage maps are available in two sizes: 162 x 104 px and 447 x 288 px. Unlike the maps on GSMWorld.com the mobile maps aren't zoomable. They don't seem to be as up to date, either. The ATT map on the mobile site is completely messed up, only showing ATT coverage in Salt Lake City, Utah!
I also had a bit of trouble finding both the operators I was using as they were listed under different names. Einstein appears as Airadigm Communications and iWireless as Iowa Wireless. It would be a big help if GSMWorld included each operator's Mobile Country Code (MCC) and Mobile Network Code (MNC) in the results or better yet provided an MCC/MNC lookup. Unlike names these codes do not change. Of course you would have to know what MCC/MNC to search for. Some phones, like the Motorola Z8 can display the MCC/MNC without additional software and for those that don't you can use a Java or native application. On the N95 I use a great freeware application called PhoNetInfo.
Besides coverage maps GSMWorld.mobi offers press releases and an events calendar although like the maps they aren't current.