Sounds like supergenpass which was implemented in Javascript as a bookmarklet, which is pretty good idea, except that any website with malicious code could grab your master pass from the DOM, and then they could use that for your account on any other website.

It looks like pwdhash works around this by functioning as a browser plugin (making your master password in accessible from the DOM) but I'd still be slightly worried about browser exploits allowing malicious sites to get at your master password.