I am a big fan of Desiring God. Mostly because of the excellent teaching offered by John Piper, who started Desiring God Ministries out of the conviction that "God is most glorified in us, when we are most satisfied in Him." In their newsletter this week, Piper goes over a brief summary of the reasons Desiring God exists. I appreciate Piper's humility on his ministry which is stated in the first paragraph of the article.
I also found it helpful in the article, a list of books and subjects they cover. It is by no means an exhaustive list of teachings on those subjects, but it is a sampling of what Desiring God Ministries offers. I also appreciate their policy of whatever-you-can-afford for purchasing materials. They truly wish to serve the body at large and trust God to provide for them. Their example of this kind of service has indeed spread to other online sites of Christian ministries as well.
So, if you've never heard of Desiring God Ministries, or if you just want to catch up, feel free to the check out the link above.