Runes of Magic's Official site – The official site is, well, the official site. Having said that, it actually pays to keep up with the news announcements, technical issues, contests, and various Item Mall specials they may have going on.
Official Forums – Most of your typical forum necessities are covered here. They do a really good job of posting patch notes, and it is for that reason alone that you should be visiting. If there's a change in how something operates within the game, it will be in there.
Facebook – Again, like Twitter, RoM has different Facebook accounts for the different language servers. I like to keep tabs on the US and International accounts. Both are in English, and it's an easy way to socialize with a wider player base, especially if you have characters on US and International servers.
YouTube – Unlike Twitter and Facebook, you only need to bookmark one YouTube account. It's mainly a repository for all the English and German language videos that can already be found on the respective official sites. Still, it's sometimes a bit quicker to find and view the videos.
Runes of Magic Wiki – Here is your great base of operation for most everything how-to. A great place where beginners can get acquainted with basic game mechanics, up to more advanced charts where players can theory craft all the different rune combinations.
Getbuffed.com/runesdb.com – Some prefer one over the other, but they are both great resources for items, NPCs, weapons, armor, you get the picture. It's very much in the same vein as Wowhead. They both offer up some nice tools to scrape character information too, for sharing profiles offline.
Curse.com – RoM uses the Lua scripting language to allow further customization of the user interface and game functionality through addons. You can add damage meters, increase mailbox functions, and a number of other things. You'll currently find over 300 addons to enhance your gaming experience. If you like coding, you can create your own addon to share with the community.
Twitter – The official US Twitter account for RoM. I've also discovered GM Eres' account hiding in a box that was buried in a sea of snow. Keep in mind that there are separate accounts for each location. You can find other official game accounts via the US one, or by going to your respective server location's main page.
That about wraps it up. It's not everything that's out there, but it is most of the major resources and covers about anything you'd ever want to know. If you do have a tip or three, you can drop me a line at jeremy[AT]massively[DOT]com