So you want this to work on any site? I don't know how.

But if you just want to bookmark each site, it would be reletively easy to just save the source on the login page, then change the METHOD=POST in the form to GET, like someone said, then just run the page from your hard drive and bookmark the URL after you login. It wouldn't be as quick as running a program to do it for you, but it'd only take a couple minutes.