I’ve used the Mindterm SSH Java and it works fine for me. Unless you sign the applet, however, you won’t be able to use it to access any site other than the one hosting the applet. But if you have a shell account, you can log into it and then ssh to another site.
One reason to use the SSH applet is as a backup in case you need to access something but can’t find a machine with an SSH client. Most any recent browser will support Java applets. (On the other hand, you do have to be wary about using a public computer; a keystroke logger could capture your password.)