Jürgen Hubert, author of the exceptional D&D campaign setting Urbis, has started a thread on EN World to be a cartography workshop. It’s really taken off, with many amateurs and professionals chiming in with neat techniques and step-by-step instructions on how to use various graphics software to make striking and useable maps. Not only that, but you may also learn the well-guarded secrets of the mysterious society of Photoshop users:
I just wanted to tell you THANKS for this thread. I've been begging someone to show me how to use Photoshop for years and all I've ever gotten was a suggestion to just start playing around with it and figure it out for myself. You've taught me more about photoshop in one night than dating four animators and two years of film school ever did.
Be sure also to peek at the still rather tiny stub Mr. Hubert has created on RPGnet dealing with the same topic.