The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- JOSM (1 member)
- JQuery (3 members)
- J K Huysmans (2 members)
- Jane Austen (1 member)
- Jason Scott (1 member)
- Java (3 members)
- John Naughton (1 member)
- Jokes (1 member)
- Joseph Fraser (2 members)
- Joseph Whitworth (2 members)
- Journalism (1 member)
- Joy (1 member)
- Kevin Kelly (2 members)
- Kevin Rudd (1 member)
- Keyboard vs. pen (4 members)
- Kieran Benton (2 members)
- King's Square (1 member)
- Kings Square (2 members)
- Kitchens (2 members)
- Knives (2 members)
- Kobo (6 members)
- Kobo Mini (2 members)
- Kohana (7 members)
- L10n (2 members)
- LOCKSS (2 members)
- LaTeX (4 members)
- Lab notebook (8 members)
- Landscape (2 members)
- Languages (2 members)
- Laptops (4 members)
- Larry Foley (1 member)
- Lawrence Lessig (1 member)
- Leaving Canberra (1 member)
- Leaving the office (1 member)
- Lectures (3 members)
- Legs (2 members)
- Lenovo (3 members)
- Libraries (9 members)
- LibraryThing (2 members)
- Libre (1 member)
- Lighting (1 member)
- Lightweight markup languages (3 members)
- Links (5 members)
- Linux (8 members)
- Linux-voice (0 members)
- Category:Linux (0 members)
- Literature (2 members)
- Little Creatures (3 members)
- Living (3 members)
- Local history (1 member)