As a part of the Transit Appliance Project, I’ve been evaluating what transit agencies have real-time data; here’s a (non-exhaustive) list, focused primarily on U.S. agencies (note: these are only agencies that provide an API, not any agency with a real-time feed): TriMet, Portland, OR, custom JSON[-P] feed, XML may also be available. GTFS-realtime is also available. San Francisco Muni, via NextBus. Bay Area Rapid Transit (BART), San Francisco, CA, custom XML feed.