Standard Ebooks is described as '"Standard Ebooks is a volunteer-driven effort to produce a collection of high quality, carefully formatted, accessible, open source, and free public domain ebooks that meet or exceed the quality of commercially produced ebooks' and is a Ebook Library in the news & books category. There are more than 10 alternatives to Standard Ebooks, not only websites but also apps for a variety of platforms, including Android, iPhone, iPad and Android Tablet apps. The best Standard Ebooks alternative is Internet Archive, which is free. Other great sites and apps similar to Standard Ebooks are Project Gutenberg, LibriVox, Myne (eBook) and Littler Books.
Julie of Global Grey ebooks formats public domain, out-of-copyright ebooks and makes them available for free download. The site has been operating for more than 10 years and currently has more than 2,000 ebooks available.
BrainAB provides an accessible archive of classic books for students. The library is searchable by title or author and downloads are freely available in Apple ePub, Amazon Kindle and Adobe PDF formats.
Global Grey and Standard Ebooks both focus on the quality of the reading experience, rather than the number of e-books. As a contrast, Project Gutenberg and the Internet Archive--and this is not a criticism of them--both seem to focus on the number of books, but don't make them easy to read (eg with curly quotes). Global Grey and Standard Ebooks do use curly quotes, among other things. So I'd say they have similar goals.