Coworking is a growing concept for shared office space, particularly suitable for web workers, contractors, and independents. Some charge by the hour, some charge by monthly rent.
Google Group for coworking - arguably the best place for leads / discussions about coworking
Coworking wiki - arguably the most comprehensive wiki about coworking
http://blog.coworking.info/ - Coworking community blog
Where you can cowork [this is an incomplete selected list of cowork locations I've found]:
For longer-term commitments and a permanent office space, good options include HQ and Regus. This is great for when you don't need a whole office building, and want an office space for 1 to 5 people.