Ajaxterm is a web based terminal. It is incredibly easy to install it’s great to have in a bind.
For instance, if you were to lock yourself out of your server, you could re-enable screen sharing with this terminal. It’s also useful for when you don’t have a computer near that has a Terminal application but you need to do some quick work on your server.
AjaxTerm is free and can be downloade here.
(Thanks Nicholas for the heads up.)