Guide to Early Church Documents From the Internet Christian Library.
The Early Church Online Encyclopaedia Document Page (comprehensive index of Western religious literature, 1st-15th Centuries)
Marc Goodacre's New Testament Gateway (highly recommended, esp. the All-in-One Biblical Resources Search)
Torrey Seland's Resource Pages for Biblical Studies
The EIKON Image Database for Biblical Studies (Yale Divinity School)
Christian Classics Ethereal Library (massive collection of texts)
Fathers of the Church (lots of texts)
Other resources available from Crosswalk.com include: