I have just answered a similar question on ResearchGate website and thought that it could also be beneficial for the Academia.SE community to share my answer here as well (as a wiki). Obviously, corrections and contributions are very welcome. Please see my answer (initial wiki post) below.
There exist a significant number of repositories that, in one way or another, contain open source lecture notes and similar or related information materials.
First and foremost, large open courseware repositories contain information on various courses, for many, including course notes. For example:
Open Course Library by Washington State Board for Community and Technical Colleges;
repository and search engine by The Open Education Consortium.
The second significant source is represented by a multitude of repositories of open source textbooks on various subjects. Such repositories include:
Open Textbook Library by Open Textbook Network;
MERLOT repository by California State University System and its partners;
Open Textbooks repository by Community College Consortium for Open Educational Resources;
Open Access Textbooks project by Florida Distance Learning Consortium;
College Open Textbooks search engine by The College Open Textbooks Collaborative;
Finally, additional stream of information can be traced to individual university departments, groups, and labs. Usually, such resources are quite narrow in their coverage and might apply to specific areas of a discipline or a course. For example, see the ETH's open source course notes on advanced methods and strategies in organic chemistry. Also, many (IMHO, most) individual professors share their own course notes on their personal university-affiliated websites.