The Seattle Public Library now has digital collections online. SPL card holders can access OverDrive (including audio books), Safari and netLibrary collections.
Lots of DRM involved, of course, and Mac users have no options for using the audiobook collection or books only available as Mobipocket eBooks. Many print titles are available in Adobe eBook form, and Adobe Reader is available for Mac. I was able to "check out" a book from OverDrive, which uses Adobe Reader whole-hog for DRM purposes. I assume I have to "check it in" again in order to free up an item in my 6-item quota, but I'm not quite sure how to do that.
I can't get netLibrary to work at all with a Mac, with Safari (the web browser) or Firefox, despite having Adobe Reader installed. (The AR 7.0.1 updater said something about fixing viewing inside the Safari browser, which is what netLibrary tries to do, but I couldn't get it to work even after the updates.)
The Safari collection can be read directly in a browser, no reader and no "check out" required. The site is a bit slow, and every clickable thing reloads the entire page (on purpose?), but it's otherwise useful and good to have. This is probably the only benefit I'll see from this for a while, and then only because I have a use for referring to technical books online. It's an excellent complement to SPL's aging technical book collection.