Westminster Bookstore is, hands down, the lowest-price, best-selection on-line source of high quality Christian books from an informed, Reformed, and academic perspective. Their blog partnership program is an amazing way to accrue credits towards free resources. It is pretty simple: click-throughs to anywhere within the WTS site earn gift certificates. And if you like books, it is a fantastic deal.
After a brief hiatus, the WTS guys have re-opened the consideration process for new prospective blog partners. You simply need to register your website with them. Check it out to get started.