CCEL - At Christian Classics Ethereal Library you can find just about anything related to church history. For example, there is good information about both the Church Fathers and hymns.
Hall of Church History - If you want to look at writings from across the spectrum of Christian history (Catholic, Eastern Orthodox, Reformer, Puritan, Arminian, etc.), this is the place.
John Owen - This is pretty straightforward. This site offers many of Owen's works and information about him.
Jonathan Edwards - Similar to Owen's site, you can find both information about Edwards and many of his works at this location.
Monergism - The best aspect of this site is the wide variety of theological articles and essays from a Reformed perspective.
Reformed Reader - As a Baptist, I love this site. It offers just about any important statement of faith from the history of the Baptists.
Spurgeon Archive - If you have an interest in the "Prince of Preachers," click on this site to find out about him and read his writings.