The Reddit Atheism subreddit.

If you are not used to reddit, the user interface takes some time to get used to.
Tools that I found useful are the filters (the colored checkboxes on the right hand side) and the tabs just under the title.
The table of contents for each tab is particularly neat.

The Reddit atheism subreddit has 1.9 million subscribers. Ken states that this subreddit is the largest atheist community in the world.
see: http://www.reddit.com/r/atheism

The FAQ for the subreddit provides concise definitions and explanations of terms (for example, it explains the differences between "atheism", "theism", "agnosticism" and "Deism" among others. It is a clear exposition of how those terms are used in the subreddit. It also provides the subreddit guidelines.
see: http://www.reddit.com/r/atheism/wiki/faq

The Gems section has a marvelous list of links. Paraphrasing Ken: this list is a collection of high quality postings from /r/atheism and is very captivating.
see: http://www.reddit.com/r/atheism/wiki/gems

For recommended reading, see: http://www.reddit.com/r/atheism/wiki/recommended/reading and for recommended videos see: http://www.reddit.com/r/atheism/wiki/recommended/viewing