Mingle2 is a membership service of which users pay to be a part.
It was originally a service called Just Say Hi, but the site transitioned to Mingle2 in 2008 and now works with many populations of singles.
Some mobile apps will match you with people based on criteria, including age, gender, and geographic proximity.
More traditional sites may offer anything from a simple search to a highly specific advanced search.
First Met, formerly known as AYI, is an online dating service that can be accessed through a mobile or Facebook app and on the web.