I mentioned John Piper's resources here before, and was reminded again of John's resources by Nicholas Batzig.
In my opinion you'd be hard-pressed to find better Christian resources for growth in the knowledge of God or how to live your life to the fullest for God, than the resources of John Piper made available at Desiring God.
John's general Resource Library can be found here. You can also make use of his various RSS Feeds and Podcasts and/or Email Subscriptions.
John also has sermons available. Many sermons are available in text, audio, and video.
Apart from the many other resources, apart from his online audio sermons, I would reckon his books to be the top resource at Desiring God. Most of these books can be downloaded as PDF.
Just to remind you, all the resources I mention here are FREE! All you have to pay for is download time. Isn't that great!