They've got a huge database of available singles, their site makes it easy to connect no matter where you're from (including if you're traveling and wanting to meet someone), and it's just plain free (unless you want limit who looks at your profile, and other non-essential features).
I had the most luck on Ok Cupid while in France with actually meeting people to have a relationship with (a surprising 100% success rate), although admittedly it did take some weeding through, and I most definitely recommend hacking your profile to meet someone fantastic.
I didn't have a ton of success with English-version site, which basically lets women join for free and decide who to contact, while the men pay to display themselves in the hopes they'll attract a gal's attention.
In France however, this is by far the most used and conversed about dating site; pretty much every "mec", or guy, that I spoke to while living in France mentioned they'd used the site at some point.
I found Adopt Un Mec more fun in French, where I could detail my needs and wants in a partner with a very tongue-in-cheek formulation, with headings like "Regional Products," "New on the Shelf," and a list of women that I'd be in competition with for any one gent's attention.