There are about 5 of them but some of them have little quirks or extras
One of them ignores the "watch this topic" setting. Indeed, if you are watching a topic and use quick reply, it will change it so you are not watching it so you never get notified of replies.
Another wont work with the message icon mod.
Some are just plain quick reply boxes with a button. Others have quote, smileys and all sorts (which sort of takes away the idea of quick reply).