The Hathi Trust is a partnership of major research institutions and libraries working to ensure that the cultural record is preserved and accessible long into the future.
Internet Archive a 501(c)(3) non-profit that was founded to build an Internet library. Its purposes include offering permanent access for researchers, historians, scholars, people with disabilities, and the general public to historical collections that exist in digital format.
Project Gutenberg "Project Gutenberg is the first and largest single collection of free electronic books, or eBooks. Michael Hart, founder of Project Gutenberg, invented eBooks in 1971 and continues to inspire the creation of eBooks and related technologies today" -- Their Website.
Open Content Alliance "The Open Content Alliance (OCA) is a collaborative effort of a group of cultural, technology, nonprofit, and governmental organizations from around the world that helps build a permanent archive of multilingual digitized text and multimedia material" -- Their Website.
Online Books Page from the University of Pennsylvannia Library.
iBiblio "an online public library with freely available software and information, for topics such as music, literature, art, history, science, politics, and cultural studies" -- Their Website.