EHarmony is the world's largest online dating service. It's a bit of a catch-all category, but I believe it has some distinguishing features. To begin, you can use eHarmony to look for people based on religion, ethnicity, and other factors. Second, they have a plethora of filters to assist you in narrowing down your search. Third, they provide relationship guidance and tools.
Eharmony was one of the first dating sites to use compatibility matching technology, which matches you with people who share your values and lifestyle interests. The company also provides its members with social networking features such as profiles, photo galleries, blogs, and chat rooms where they can discuss relationship-related topics.