Bing Maps, a service of Microsoft, began offering public transportation directions in 2010. Data for a handful of public transportation agencies is included in Bing Maps. Bing Maps ingests data in the General Transit Feed Specification (GTFS), as Google Maps does. Bing Maps publishes information about the Bing Maps Transit Partner Program, which offers transit agencies the opportunity to sign up for the Program and provide their data to Bing Maps. It is cost-free for transit agencies to participate. Bing Maps is integrating new agencies, however a timeline for adding new agencies is unclear.
Bing Maps offers roughly equivalent functionality as Google Maps with regard to transit directions. There is also a mobile app version of Bing Maps. Bing Maps also includes transit directions in their API, which allows 3rd party applications to query for transit directions and other information about transit services.