The entire Septuagint, in browsable form. Learning New Testament Greek Lots of resources and this Bible Translation website. Includes both primary documents and modern commentary. It is still very incomplete, however. Classical Christianity Online has writings from early church fathers to 20th century.
Noncanonical documents is an interesting list of early Christian literature.
Bible Commentaries etc biblestudytools.net contains a number of searchable books. Most of these are public domain, but a recent noteworthy addition is Baker's Evangelical Dictionary of Biblical Theology . Still, there's sure to be useful stuff there. But the very fact that it is online makes it useful. Nave's Topical Bible .