The Prague Information Service (PIS) is centrally located and has information about concerts, performances, and other happenings in the city. They have maps and publish a monthly guide for tourists. The staff is very knowledgeable and English-speaking.

Prague Information Service
Straromestske Namesti 1(Old Town Hall)
Rytirska 31
Main Railway Station (lobby )
Lesser Town Bridge Tower (summer season)
www.prague-info.cz

This is the Czech Tourism official site with lots of information about Prague.