Very comprehensive class note website available at CourseHero. Note to be cliche, but this website can certainly save you if you have missed a significant number or classes, or you need a good supplement to your lecture notes.
You can find ever more free lecture note resources at our comprehensive list here.
Although not all of the lecture notes on this site are free, ShareNotes definitely has a decent amount of free college class notes. This site is definitely worth checking out.
Check out NoteLog, a new college class note database. An excellent resource in the event you have missed a college or university lecture.
Check out WiseCampus.com, an excellent website that rewards college students that post their lecture notes with points and the possibility to cash in those points for gift cards and other products. More importantly, college and university students can get lecture notes to the classes they missed at this site.