Twitterers who are not humans or individuals (generally speaking):
Applications, etc.
Bots
Comedy
Contests
Employment
Libraries
Literary
Movies and TV
Museums
News Services
Organizations, Companies, Blogs and Websites
Transportation
Air
U.S.
Non-U.S.
Railway
U.S.
- BART (San Francisco subway)
- Caltrain (SF Bay Area train)
- WMATA (Washington, DC subway)
Non-U.S.
Roads
U.S.
Non-U.S.
Weather
Other