Casual Kiss is, as the name implies, for those who are looking for something casual.
Soul Mate Find is geared toward people who are trying to find life partners.
While using the site is free, it does require "credits" which you earn by logging in every day or by referring friends to the site.
This site also has a mobile app available for free.
Plenty of Fish works to set you up with other members you're likely to be compatible with based on a chemistry test.
All of the sites listed here are free to browse, to set up a profile, and to communicate with others via message or email -- whether you initiate the contact or the other person does.
Date Hookup's criteria include hair and eye color, education, personality (shy or outgoing) and occupation.
One advantage is that it's been around since 1997, long enough to have a base of over 5 million users, even though it is a lesser-known site.
You can also view the site in Spanish or Portuguese, which may be helpful if you are bilingual.
You'll also get free relationship coaching via blogs and a popular message board.This site is for people looking for dates and also for casual flings. Date is another one that uses an algorithm to match users.