There are actually a lot of sites like Ashley Madison because it's simple to create a cookie cutter site, put a sexy cover image on the front, and make some money.
But yeah only a few of these sites actually work well to find affair partners.
Here are my observations.
1. Victoria Milan
Pretty much an Ashley Madison clone but with a few key differences. Like membership fee instead of credits, virtual gifts, and that sort of thing. Not as many members but still discreet, anonymous, photo editing features, private galleries, etc.
2. Adult Friend Finder
Still a reliable site to find sex partners, just overwhelmed with men, and with a lot of flakes who aren't going to meet you. Still, great place to meet many people of all different preferences, orientations and lifestyles. Forums, chat, and blogs. Like the Facebook of the underworld.
3. NoStringsAttached
4. Heated Affairs
Heated Affairs and NoStringsAttached were both in the AFF network but NoStringsAttached has since shut down, while HA remains open for now.
5. Flirt.com
6. BeNaughty
Flirt.com is seems more app friendly than the usual hookup dating site IMO,
and at one time was owned by Cupid PLC, but have since been taken over by Together Networks. They also
BeNaughty.com and WildBuddies.com. About 70 million users between all of them. At least these sites are app friendly, simple, look good and work fast.
7. Gleeden
A European site that matches together unhappily married or couples in open marriages. What's cool about it is that it is geared towards women, rather than men, which is Ashley Madison's problem.
8. MaritalAffair
Cheating site but more popular in the UK than the US. Still, nice services, and all profiles are moderated for quality and genuineness.
9. Seeking
Kind of a luxury sugar dating thing, but very discreet and low-key. At the same time, offers verification of identity AND millionaire status.
10. Flure
More of a fantasy, anonymous confession sort of hookup site. Not specifically about affairs, but surely affairs are one of the top fantasies exchanged. Personal ad style posting.
11. OneNightFriend
Kind of a new site but reminds me of Flirt.com. They are probably in the same network of cheating sites. Nice one, friendly, easy to use interface. Kind of small membership though.
I'd also suggest that if you're after One Night Stands, you'll find just as many on the mainstream dating apps like Tinder, Bumble, Hinge, POF, etc.
I've never known ANYONE who complained about seeing their affair buddies a second time.