Department of Engineering
A mapping help site which provides helpful tutorials, forums for mapping help and a testing zone to playtest beta version maps for bugs before release.

The Snark Pit
Everything from tutorials to reviews with extensive user comments.

Source SDK Documentation
Contains information on the Source engine and working within it.

TF Mapped
Everything you need to make TFC maps. Lots of great tutorials, tons of information, forums and a great IRC channel.

The VERC Collective
One of the largest collections of tutorials I've seen along with files, ultilities and entity lists.

The Wadfather
An amazing site with lots of textures and environment maps.

Burnt Popcorn
Home of mapper FryGuy, a friend and very good mapper.

JobsMaps
Has made lots of skill maps that look great and are fun to play.

Mr. Suko's Map Archive
Home of mapper Zouave who has not only made a lot of good TFC maps, but is also a mapper on the Fortress Forever mod.

Mulchman's Domain
Mulchman's site to showcase not only his maps, but software and web development projects. Mulchman is on the Fortress Forever mapping team.

Righteous Anger Maps
Maker of open fort, and uruic.

Trepid
Web page for Jon and Jesse, known as the Trepid mapping team. Their site also has forums, tutorials and map reviews! They are also part of the Fortress Forever mod team.

Fortress Forever
Fortress Forever is a mod for Half-Life 2 which I'm on the mapping team for with many talented individuals.

Half-Life Rally
Rally styling racing for Half-Life 1, with a Half-Life 2 based version of the mod currently under development.

The Catacombs
The Catacombs provides extensive coverage on the clan scene of TFC, backed by very active forums.

Invade-Defend
The .id guild is dedicated to supporting custom invade/defend style maps and providing a community where players can come together to share ideas and form friendships.