The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Christian (4 members)
- Club Der U2-Liebhaber (1 member)
- Codes (43 members)
- Cologne (4 members)
- Comedy (4 members)
- Communications (4 members)
- Computer (452 members)
- Concord (15 members)
- Conspiracy (53 members)
- Conventions (3 members)
- Converter (11 members)
- Copyrighted free use (1 member)
- Corporations (14 members)
- Countries (1 member)
- Cowsay (54 members)
- Crypto (19 members)
- Cybernetics (1 member)
- Cyborg (1 member)
- DIY (7 members)
- Debian (30 members)