Openly Available Sources Integrated Search (OASIS) is a search tool that aims to make the discovery of open content easier. OASIS currently searches open content from 97 different sources and contains 385,629 records. OASIS is being developed at SUNY Geneseo's Milne Library.
OpenStax is an open textbook publisher based at Rice University. Its collection mainly focuses on general education classes, and their textbooks are peer-reviewed. Additionally, they have at-cost print versions of their textbooks for sale.
The Open Textbook Library has a collection of over 100 peer-reviewed and openly licensed titles in a wide range of disciplines. It is supported by the Open Textbook Network, a collaboration of library partners from colleges and universities across the country.
Funded by British Columbia's Ministry of Advanced Education, BCcampus OpenEd has a collection of 180 peer-reviewed open textbooks covering Sciences, Trades, Business & Management, Liberal Arts & Humanities, Social Sciences, Upgrading Programs, Health Related, and Recreation, Tourism, Hospitality & Service.
SkillsCommons has a collection of open career and technical education materials. SkillsCommons’ mission is to accelerate the democratization of education for all through open educational services and resources enabling individuals, communities, educational institutions, organizations, and businesses to prepare people for successful employment in the 21st Century.
The MERLOT collection consists of tens of thousands of discipline-specific learning materials. All items have been contributed by the MERLOT member community, who have either authored the materials themselves, or who have discovered the materials, found them useful, and wished to share their enthusiasm for the materials with others in the teaching and learning community.
Additionally, you can find peer reviews from CCC, CSU and UC faculty on its associated site COOL4Ed.
Using Google Advanced Search is a great way to find resources by license type. Here are directions on how to use Google Advanced Search to find materials with open licenses.
"Open Education Week: Finding Open Educational Resources" by David Wiley is licensed under CC BY 4.0