I use the Loginza and it works very well.
For Q2A, SLonoed wrote a nice plugin for loginza.
How to use it:
Download the plugin to the qa-plugin directory.
$ cd qa-plugin
$ git clone git://github.com/SLonoed/Q2A_loginza_plugin.git
Configure the the file qa-loginza-login.php in the plugin directory to set it. Follow the tutorial here.
No need to overwrite the
nav_list($navigation, $navtype) function from my own experience.
Additionally, if you know some HTML/CSS, you can customize the button for login by yourself (as shown on this site).
One of my concern is that Loginza may stop providing service or start to charge money some day. I figure out that this is not that dangerous (still very bad. I hope that Loginza is free and live forever) since the user database is on my own site. The users can still log in their accounts after some struggling (dirty, but there is a way out):
Update and confirm the Email address in the account profile.
Set (reset through "forgot my password") their passwords.
Then they can log in my site without Loginza using their Email/passwords.
Hence, I am glad to use Loginza now.