The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- The Co-operative Food Shop (2 members)
- The Day We Fight Back (1 member)
- The European (1 member)
- The Guardian (1 member)
- The Mystery of Edwin Drood (1 member)
- The Queen (1 member)
- The Thin Blue Line (1 member)
- The Unknown Craftsman (1 member)
- Themes (2 members)
- Theo Aronson (1 member)
- Theo Coulthard (1 member)
- Things (5 members)
- ThinkPad (1 member)
- Thomas the Tank Engine (1 member)
- Thunderbird (1 member)
- Tim Berners-Lee (1 member)
- Tim Brook-Taylor (1 member)
- Time (1 member)
- Timekeeping (1 member)
- Tom Bihn (1 member)
- Tom M. Wilson (1 member)
- Tools (5 members)
- Topics (1 member)
- Torrents (1 member)
- Trains (3 members)
- Transport (15 members)
- Travelling (2 members)
- Travels with Charley (1 member)
- Treenails (1 member)
- Trees (3 members)
- Trello (1 member)
- Tropicana (1 member)
- Trove (1 member)
- Truth to Materials (1 member)
- Tuggeranong (4 members)
- TwentyTen (2 members)
- Twig (1 member)
- Twitter (1 member)
- Typesetting (2 members)
- Typing (1 member)
- Typography (2 members)
- UNC (1 member)
- URI (1 member)
- URIs (1 member)
- URLs (3 members)
- UTM (1 member)
- UX (1 member)
- Ubuntu (7 members)
- Unesco (1 member)
- Unit testing (1 member)