I was trying to find A.E. Brooke's critical edition of Origen's commentary on John today, and the UChicago catalogue provided a link to Hathi Trust, which I had never heard of before. It's another digital repository that will be worth bookmarking.
As far as I can tell they provide open access for out-of-copyright books, and have more that are in-copyright (I'm not sure of the viewing availability of this material). There are, however, items available in fulltext on Hathi Trust that are not available on Google Books. The commentary I was looking for was actually scanned for Google Books by the University of Michigan, and it comes up in a Google Books search, but for some reason there is only partial viewing available on Google Books while there is full access on Hathi.
Hathi, like Google, seems to be working with a number of research libraries to sustain its archive. The University of Chicago is a member institution, but from what I can tell UChicago has not to this point actually deposited any of its digitized holdings into Hathi Trust (nor have most of the member institutions, it seems). While I'm not very well-educated on the major digitization projects going on these days, it strikes me that compared to other ventures, Hathi Trust is much more of a cooperative and careful enterprise, and is deeply invested in working with research libraries for the purpose of quality archiving. Looking through their site, you don't get the impression that this is a mad dash to throw a bunch of stuff up there. They seem quite methodical and deliberate. I imagine this may put them in a position to be the eventual standard for digital bibliographic institutions as things move forward.
Archive.org is the other obvious place to go for a large collection of digitized books, and of course there are many other smaller ventures out there depending on your particular research area. Being aware of the diverse options is important, as the future of a single headline-grabbing venture like Google isn't set in stone, and relying on Google Books as a crutch for finding resources will leave one unaware of a whole lot more that is out there. Amongst the big players in digitization, though, I think Hathi seems the most prepared to actually make good on some of its intentions in the long-term.
You can find their most recent update here, with statistics on collection growth and current activities such as disaster recovery planning and quality control measures.