Is stack overflow uisng https://rpxnow.com/ for login in using different services. If so is it good, and does it have good (preferable free, preferable with PHP api) alternatives.
What I'm looking for is a login page wich would allow users to login using major web open-id providers + facebook connect.
i remember signing up for RPX and being able to easily find a PHP example on their site.
Also, I don't believe stack overflow uses that, but i could be wrong
No, StackOverflow does not use RPXNow. And if you choose to use it be careful to avoid its pitfalls.
I suggest you just accept OpenID, which covers many, many Providers (who uses Facebook but doesn't use one of Google, Yahoo, and (soon) Microsoft?). Besides, I expect as an OpenID supporter, Facebook will eventually become an OpenID Provider too. So I suggest you save yourself the trouble and just support OpenID using one of the PHP OpenID libraries since that is your language.
For completeness, www.clickpass.com is very similar to RPXNow. However, it also has it's detractors. http://notsorelevant.com/2008-03-11/clickpass-i-dont-get-it/