Thanks to Campus Mawrtius, one of those rare resources that makes your heart sing and (potentially) halves your time in the library.
Mischa Hooker has catalogued the Corpus Scriptorum Historiae Byzantinae; the Patrologia Graeca, and the Patrologia Latina and put links where they have been scanned by Google books. No more racing to the library to check a reference before finding that the one volume you need has been taken out by someone in the theology faculty, and who has now gone on leave for the next six months.
Although CSHB has been scanned in its entirety and is online, PG and PL are less complete. Nonetheless, a great resource.