Here are the best online editions of the writings Church Fathers that I can find and that are free. Note especially the Patrologia Latina and the Patrologia Graecae, the most complete original language editions of the Church Fathers. Unfortunately the Corpus Christianorum is not yet available online.

CCEL (English)
Fathers of the Church Series (English)
Patrologia Graecae – Version 1 (Greek)
Patrologia Graecae Version 2 (Greek)
Patrologia Latina (Latin)