Since there isn't one already, I'm listing here workshops that I found interesting and helpful in learning more about drawing. Feel free to post your own as well!
ImagineFx: (The files listed have the support files as well so you can paint alongside with the workshop. Really helps with developing your own art skills)
www.imaginefx.com, they have a monthly magazine for fantasy art.
This one is about making a monster by using a photo by Dave Rapoza. His website is www.daverapoza.com
This one is about painting a Kraken by Bart Tiongson
This one is about learning how to paint in a Frank Frazeta style.
I'm working on a piece now, and when I'm done I'll show how I did it step by step with gimp.