OKCupid is one of the oldest dating sites, but it's still popular thanks to it's user friendly attitude, a smart matching algorythm and the active users' community.
OKC is a slightly mutated child of a website called SparkNotes which featured tests and quizzes and had a match test which later evolved into the matching system used by OKC. The site was free from the beginning, but got some extra features the more demanding users can pay for.
OKC is a dating site, so the most important thing about using it is creating one's profile and browsing through other people's profiles on the search for a kindred spirit. Making a profile is easy, and the site is liberal about what the users post - it can be a lenghty poetic description or just several witty verses or quotes. The photos, however should contain the user (at least the main profile pic) in a clothed version. Browsing to find a friend is more tricky, with over a million users one might requie some help with finding an appropriate person. That is where the matching algorythm comes in. Each user who wants to take his time and answer several (or several hundred) questions will have his personality profile created, and also will see the percentage of similarity between him and other users. The questions regard views on relationships, sex, religion and general lifestyle and can be added by users too. The system works quite well, and helps the users find people with a high match% or those who are likely to be good friends, or even the people the user has the least in common with. That way, a polyamorous bisexual feminist will not get a suggestion to meet a traditional, marriage oriented Mormon, but will see profiles of people with similar views.
Once you take a look at someone's profile, the owner will see you did it and will be able to visit back. Another place where people can discuss things and also meet is the forum, filled with opinions, rants, success and failure dating stories and threads like 'the user above me is...'.
Apart from the matching robot, we can also use a regular browser, showing people who are near a chosen location, or who have certain preferences (guys who like girls, girls who like guys, and many more) the highest (or lowest) matches, or those who use keywords the user likes.
The basic features of he sites are free, and you can browse and get in touch with people without any frustrating messgaes like 'you got a letter from the person you like, but have to pay to see it'. However there are some extra things you can pay for, such as a bigger mailbox, the ability to stalk profiles without beeing seen, or more advanced search options.
The site is modern and easy to use, clear ikons show the way to different sections and the users' profiles are easy to find and read. Profile customization is not possible (apart from the written content and photos, of course). The IM window is big enough to read easily, but doesn't obscure other important parts of the site.
OKC is a great site for finding friends, or even dating for people who don't take the whole dating thing too seriously and are not offended to see profiles of people with various orientations and worldviews. This site is open and independend enough to tempt eccentric people, but not too niche, so people with traditional ideas feel well on it, creating a meeting ground for people of many different subcultures. The matching system is fairly accurate, but if you don't find a soulmate, you can have fun wih quizzes or just talk to people and make non-romantic friends. It is also a source of knowledge for sociologists, psychologists and people who are simply curious - the OkTrends blog (http://blog.okcupid.com/) analyses the results of questionnaires and puts it into entertaining charts.