Hot diggity biscuit! A software engineer at Google noticed there was a Facebook group called “Get Google Magazine Search to provide a list of indexed titles” (I wish I had known about that one, I would have joined!) and took some time to actually make a page on Google Books that lists all the available magazine titles.
The list of titles is at http://books.google.com/books?as_pt=MAGAZINES&rview=1. This is a cover/grid view; there’s also a list view if you prefer. You can filter the magazines displayed by those which have full view only and those which are public domain only. I didn’t see any magazines that were listed as public domain only, but when I restricted my results to those magazines which were full view, I got several dozen titles, from The Alcalade to Yoga. Some highlights (to me anyway): Billboard, Dwell, Popular Mechanics, and Popular Science.
This is great! I’m so glad that you can see all the magazines that are available on Google Books (and I’m astonished and happy there are so many!) Now for the next possible trick — how about being able to search a set of magazines at a time? (Instead of just one of them or all of them?)