Rinetd is not really a proxy, but it can accomplish almost the same thing. Its a TCP relay, which can listen on an ip address and port and relay the transmission to another ip and port, and back again.
Why is this useful? I'm using it as a simple way to replace NAT over a VPN, which I just couldn't get to work. More specifically, I have Verizon FIOS, and since, in my opinion, Verizon charges way too much for IP addresses, I only requested one. How can I route additional IP address to services behind a firewall on the FIOS IP address? With the use of a VPN which can connect a remote system to the LAN, and then relay the public IP on the remote system to a private IP on the other end of the VPN LAN. Yes, its that simple.
I'm not sure what type of security or performance issues might be encountered, so I'm planning to doing some testing. In the meantime, your thoughts and feedback are appreciated!
More information: Rinetd page at Docunext