City of Heroes/Villains Official Website:

City of Heroes – The official City of Heroes/VIllains website

City of Heroes Forums and Community Oriented Websites:

City of Heroes Official Forums – The official City of Heroes/Villains Forum.
City of Heroes Collectible Card Game – Get the trading card game for City of Heroes.
Coh Writers Guild – A collection of fan fiction and fan art submitted by the players of City of Heroes/Villains.
Paragon City Comics – A great collection of comic books created by players of City of Heroes.

City of Heroes Facebook – The official facebook page for the game – City of Heroes

City of Heroes Knowledge Base Websites:

CoX Stratics – Stratics websites are always a great source of information..
COH Guru – Potentially a great source of information and guides on City of Heroes. Last time I looked though, all the content seemed to be missing, so could perhaps be still under construction.
City of Heroes Wikipedia – City of Heroes page at Wikipedia.
City of Heroes Wiki – Another City of Heroes Wiki
City of Heroes Coldfront – City of Heroes information resource. Lots of info here.
GameFAQs – Another good source of info on a lot of games.