Crash, the creator of Probed and one of the Senior Admins at TF2Maps.net, has been working on a series of up-to-date, highly informative mapping tutorials for Hammer. He's not finished making them, and plans to cover as many topics as possible, from the basics of the Hammer editor, to gameplay theory to art-passing and art detailing techniques. If you're at all interested in learning how to design a TF2 map, or are just looking to brush up on your skills, they're worth a look.