Obsidian Portal: Campaign website and wiki. Useful way of sharing setting and reference information with players between sessions. An alternative is the Scabard campaign manager, but I have yet to try that in anger.
West Marches: Interesting and different approach to fantasy gaming; a sandbox with no regular schedule or party membership. Very reminiscent of the way the first D&D campaign I played in was run, except that had no regular GM either. (Ars Ludi)