[i80and]

It's pretty darn easy to integrate, too; I used it for a toy project, and it was really quite pleasant!

[TazeTSchnitzel]

Yep. I've used it for a few sites because it's simple to implement and saves me the hassle of making my own registration system. Actually, I've used it for every site I've made requiring login, except for one which sadly couldn't use it (targeting the Nintendo 3DS web browser, which is legacy WebKit).