Desiring God Ministries is the public outlet of all things John Piper. For a long time, they have been willing to work with people to get resources in the hands of those who might not be able to afford them otherwise. Lots of stuff is available at little or no cost if you’re willing to work with them. They are not just giving everything away, but if you have a need they will provide you with Piper books and other resources. They are found online at desiringgod.org. You may also want to check out the Desiring God Blog.
Recently everything John MacArthur has become available. At Grace To You (GTY.org) you can read, listen to, or buy a CD of his sermons – 2725 of them. MacArthur is available through radio, television and podcast, and Grace to You has it all on their website. This has gotten a lot of buzz on the blogosphere, so expect to hear sermons ripped off from MacArthur at a church near you soon!