RP 2.1
(not written by me, but I added to it...)

RP is a tool for allowing one to telnet backwards through a firewall. You have to be able to get access to the inside machine first, though, to get the client on the machine. I've only tested it on Linux. The terminal emulation now works. The real home for this utility (and the original author) is Matt Miller at Hick.org, and this link is likely more up-to-date.
(ssh port forwarding obviously obsoletes this...)