The Davis Square Community

https://www.mbta.com/developers is reasonably accessible, if you're into that sort of thing and don't mind registering for an API key. The GTFS-Realtime feed is very standard, and says where the busses _are_, but doesn't include predictions (you could make your own). The T also has what you might call a "proprietary" API (that is, it's T-specific and not like GTFS where lots of agencies provide data in the same format) that includes a specific call to retrieve predictions.