Originally a document hosting site, Scribd launched their ebook subscription service back in October 2013. They boast 300,000 titles which can be read in apps for the iPad, iPhone, and for Android. If you don't have a mobile gadget you can also read in a web browser.
After Amazon, Scribd is probably the largest ebook subscription service. They claim a larger catalog than Oyster or 24Symbols, and thanks to their catalog and global focus they probably have more subscribers as well.