Brought to you by the creators of SparkNotes.com, OkCupid combines user-generated questions and mathematics to determine members'compatibility. As a member answers more questions (of which there are thousands), the stronger his matches become. The questions range from the simple to the provocative and some are best not asked in person (unless you want to ask your date whether she thinks women are obligated to shave their legs). But hey, at least the site is completely free to use.