A good first place to look for online yearbooks is a school's official website or alumni organization, as many schools are diligent about archiving and sharing past yearbooks. Other excellent options are free websites such as Classmates.com, ThisOldYearbook.com and Archive.org, where thousands of yearbooks are available.
Prime examples of school-specific yearbook collections are Southern Methodist University and University of Tennessee, who both have exhaustive yearbook archives online. Dating back to the turn of the century to present day, they have been digitized cover to cover. Schools can often be the best resources for finding yearbooks.
Classmates.com has a collection of online yearbooks numbering in the hundreds of thousands. Therefore, it is an essential website to visit when searching for yearbooks. Its entire catalog of yearbooks is available to anyone who registers for a free account. Yearbooks can be searched by school, year and city, as well as by student name, which is a search option not offered by most other yearbook archives. Additionally, Classmates sells reprints of any of its online yearbooks to anyone who desires to own a hard copy.
Other websites have less extensive yearbook collections, but they are still worthwhile sources. ThisOldYearbook.com is organized by state and links to online yearbooks hosted by various websites. Archive.org is an online library of all sorts of media. Typing "yearbook" into its search field returns thousands of hits.