What is the best tool for creating user accounts on many different systems?

I imagine there might be a good firefox plugin for this. Basically, when linkbuilding, I've found some high PR blogs on proprietary platforms which require registration before comments can be posted. I imagine such a tool would let you populate fields inside the tool and switch between account profiles that it would populate form elements on a page with. + points if the tool can click registration links in emails using pop, like those created with cpanel Even if all it does is fill in forms, it would be a huge help. Can someone point me in the right direction?