The Harry Ransom Center at the University of Texas at Austin now has an online database of Italian opera libretti available. The database has over 3400 items and includes cantatas, serenatas, and oratorios. You can access it at http://research.hrc.utexas.edu/music/, with an information page at
http://www.hrc.utexas.edu/collections/books/holdings/libretti/.

The access page is straight search, giving you the option to search in several fields inlcuding title, call #, librettist, location, and printer. I can’t imagine something I’m less qualified to come up with a search word for, so I punted and did a keyword search for opera. I got 424 results presented in a table that shows a variety of information including item title, composer, theatre, location, and date. Click on the title of the item for a little bit of additional information including title page information and cast, if available.

I didn’t see any digitized images, but some of the items here are so old (the collection contains items that were published as early as 1600) that I can’t imagine the data about them is available many other places.