Valve Time and the Team Fortress Official Blog are both excellent resources for any Team Fortress fan that is looking into maybe making maps or weapons for the game. Most of the items and maps added into the game these days are community-contributed, and Valve has very graciously made available to the general public their in-house tools for map-making. Valve Time has a lot of tutorials and such for making maps, and the Team Fortress Official Blog has news regarding the game as well as general criteria for what makes a map or an item suitable for the Steam Workshop.
As much as I despise this site for what it’s done to Team Fortress 2 trading, it is quite a reliable guide when it comes to pricing items for sale to make sure you don’t get ripped off. While I tend to take the “trade stuff I don’t want for stuff I do want” approach, it’s still nice to have a guide for the occasional selling of a hat.