IPShutter

IPShutter lets you firewall off ports such as ssh, and selectively enable access with a one-time password. For example, if you want to log into the server from a friend's house, you pull out a list of one-time passwords and point the Web browser at a URL that contains one password. That tells the server to allow ssh connections from your friend's IP address for five minutes. Connections made in that period will stay up indefinitely.