I was searching around today for a text of Didymus the Blind's De Trinitate (I'm thinking I might commence a translation into English. So far as I know there doesn't exist one. If it goes well, I might write a theological commentary on it too) and came across the following very helpful links:
An index to Google Scans of Migne's Greek Patrology
An index to Google Scans of Migne's Latin Patrology
There's a lot of great freely available source texts in their original languages now being made available online, half the problem is always just tracking things down. This might help a little. Great work.