Local adult dating and sex services
Basically, if you're looking to hook up, this is a good option. Here you avoid some of the BS on other dating sites, as everyone knows the game going in.
Also, the site is loaded with Meet Foreign Women for Dating or Marriage If you're looking to meet foreign women, whether for dating or for marriage, you should check out this introduction site.
Things have changed from the old "mail-order-bride" days, and now these are more like introduction services, and it's much easier to screen potential women with photos, chat, etc.
To answer your question: it depends on what you mean by legitimate.
We've all seen them - the ads show a bevy of beautiful women in your area, but when you sign up, you realize that the women were just ads that were geographically targeted to your search preferences.
Even sites that may once have been good can go bad.
Ashley Madison This is getting a ton of attention as it's the place to go if you're interested in an affair. Well, if you have the means to spoil a young hottie, you'll find tons of hot women on this site looking for a Sugar Daddy.
It's not just for people who are attached, however. Some or looking for some short term NSA fun, while others are looking for an ongoing relationship with someone who can spoil them.