There are two main MTurk forums, Turker Nation and MTurk Forum (links can be found below). Both serve their purpose well and there is no reason why you cannot join both of them, if you wish. You can communicate with the MTurk community and keep up to date with what is happening. For instance, you can sometimes find out useful info such as which MTurk requesters have a bad reputation. You can also use them just to socialize and have fun. Both these MTurk forums are ‘unofficial’ in that they are not run or administrated by Amazon, and they are run very much from an MTurk worker perspective.
MTurk forums are generally a fantastic source of information for beginners and experienced MTurk workers alike. I never cease to learn from them and they are awesome free resources. People new to MTurk (aka known as Mechanical Turk or Mech Turk) might find them especially useful. To join, you just need to register for free, get yourself a username and a password, read the rules and away you go!
(I believe that Amazon do have an official forum but it is for MTurk software developers, not MTurk workers.)
The Turker Nation Mturk forum can be found at http://turkernation.com It is efficiently administrated by the firm but fair, Spamgirl. It’s always pretty vibrant there in my experience and there is a tremendous wealth of Mech Turk news, information and opinion in the discussion threads.
The appropriately named(!) MTurk Forum can be found at http://www.mturkforum.com The site has a simple and easy layout, but it is not usually quite as lively as Turker Nation in my experience. It serves its purpose well, however, with plenty of discussion threads. I don’t know who the administrator of MTurk Forum is, unfortunately. (Please feel free to introduce yourself if you’re out there?)