Champions Online Official Site -- Champions Online's official site isn't the best for detailed game information, but don't be fooled. The official site is packed with tons of lore as well as interviews with developers, producers, and community reps. You can look up people's characters, including pictures, powersets, bios, and even active missions. There's even a cool little app that functions like a superhero version of Hot or Not, allowing you to rate other characters' costumes and see what people think of yours.
Official Forums -- The most interesting fact about the official boards is the frequency of posts by Cryptic staff. The dev team and community managers do a very good job of keeping players informed as of late, so the boards are the best source for info when it comes to info straight from the horse's mouth. Also, it's a great place to find information, if you don't mind digging for it. Currently, the best guides for stats and powers are all on the forums and not on another website, so it's always a good idea to browse the forums if you've got a question about a power.
Facebook -- The official Facebook page isn't the best source for news, but it is a good way to find Facebook friends who happen to share the same love for CO that you do. On the flip side, there's also the Champions Online Facebook game, a sort of Mafia Wars-inspired game you can play on Facebook, so you can get your Champions fix even when you're supposed to be working.
Hero Games -- It's easy to forget that Champions Online is cut from the cloth of Hero Games' superhero roleplaying system. While the two games have very different mechanics for playing (obviously!), they are set in the same world and share the same pool of lore. If you're interested in more information about the Champions world, you might want to check out the role-playing books that started it all.
Champions Online Wiki -- CO-Wiki, while a work in progress, is the standard for all knowledge pertaining to Champions Online's gameplay, including mission details, crafting information, and detailed information about the various powers in the game. The wiki is incomplete, but it contains a lot of useful info. Of worthy note: CO-Wiki is the best place to find pictures of all the different custom weapon skins and how to unlock them. Also, feel free to contribute if you've got time -- the wiki can always use new editors to add content!
Champions Online Roleplayers -- Champions Online has a very large and active roleplaying community, and this is the place to go if you're looking to get involved. It can be a bit intimidating to brave the dangers of Club Caprice looking for people to talk to, and the denizens of CORP are more than willing to help someone new to roleplaying in Champions with contacts and advice on character bios, backstories, and costumes.
PRIMUS Database -- Did we mention that CO has an active roleplaying community? PRIMUS Database is the repository for information not included in Champions lore. It's a player-run wiki that houses thousands of character bios and stories. There are easy-to-use wiki templates to get your own characters featured, and you can add pictures, link to RP stories that you've been through, and all sorts of other neat stuff. If, like we do, you find that the in-game bio lacks the space you need to really be creative, you can really express yourself here.
championBuilder -- Like CO-Wiki, championBuilder is very much a work in progress. However, it is still a great resource for planning out your character, including powers, advantages, talents, and superstats. There's still some inaccuracies as far as detailed information is concerned, but it's still the best tool you can find for planning your build out in advance.
Crime Computer 2.0 -- The in-game Crime Computer is a good way to find new missions to tackle, but the online, fan-made Crime Computer 2.0 gives much more info than that! It'll show every mission your hero hasn't completed, including information on mission perks, contact locations, and much much more. There is even a number of filters you can apply to your search to include missions you can't pick up anymore, missions over your level, and exclude 5-man lair missions that are (usually) too tough to solo.
Champions Online Official Twitter -- The official feed is a great place to get CO's news without having to constantly refresh the website. The CO team mostly uses the feed for official front-page news, rather than random community info or commentary. Fortunately, many of the CO team members frequently use Twitter, including:
StormShade (Shaun Brodie) - Lead Community Rep
Arkayne (Randy Mosiondz) - Lead Designer
GM_Destra (Noel Holmes) - Lead GM
Alivet (Diana Sherman) - Community Content Writer
Chronomancer (Bill Roper) - Former Executive Producer
While this is a pretty comprehensive list of places to sate your need for Champions information, our intel isn't perfect. So if you have some juicy info on fansites or information pertaining to the CO universe, give me a tip at Patrick [AT] massively [DOT] com, and we'll add your info to our database.