Stephen Carlson notes that a searchable version of the Biblia Patristica--the definitive index of scriptural quotations from the early church fathers--is now available here. It's a free tool, although registration is required. I've added it to the list of "Related Links" for future reference.
Patristic scriptural quotations remain one of the most unmined aspects of New Testament textual criticism, largely due to the enormous size of the corpus (perhaps one million total citations). Technological innovations such as this initiative, however, could go a long way towards resolving the problem and fully appreciating one of the most vital and vibrant groups of witnesses to the New Testament text.