WAW Library Catalog
Catalog for William Allen White Library.
Provides many works for free that are old enough to be within the public domain; luckily many primary texts and commentaries meet this description. To search for free full-text sources using Google Books, use the advanced search function and limit the search to "full view only." Many ancient, medieval, and modern philosophy texts and commentaries can be found. Freely available works in the contemporary and current era will be limited.
Hathi Trust Digital Library
Contains a variety of digitized e-books from various institutions. Holdings can be parsed down by selecting options for full-text, language, author, publication date, location of publication, institutional location and format. Keep in mind, many full-text books will be need to be old enough to be within the public domain.
The Perseus Project is an open source digital library that collects classical texts in Greek and Latin. A full list of available titles in Perseus is available here. Perseus also contains a Greek-English lexicon and a Latin dictionary.
ACLS Humanities E-books
Contains numerous e-books related to the humanities with some coverage of philosophy.