ICTlogy

http://www.gsmworld.com/roaming/gsminfo/index.shtml

This section provides detailed information about network operators worldwide that offer services from the GSM family of Wireless Communications. Specific details included within this section are network, roaming, services and coverage information for over 860 networks in 220 countries/areas of the world.

Developed by the GSMA.

Coveragemaps.com publishes the interesting GSM Coverage Maps, though data is only available graphically (no numbers, no spreadsheets).