Birch is a simple proxy or tunnel application with a filter mechanism for line based application protocols built on TCP. Initially it was written to add encryption in a transparent way to the IRC protocol but as the encryption mechanism is implemented in a modular way, it is not restricted to just supporting encryption. Filters can be supplied via Java's Service Provider mechanism. A CLI and a GUI are available.
sshh is a set of two daemons that allows you to maintain a TCP connection if all you have available is an HTTP proxy. It is similar in functionality to httptunnel, but has a few more features. Once you have it set up, sshh will allow you to make connections forward through the proxy or backwards through it, so you can open connections in either direction.
pwnat, pronounced "poe-nat", is a tool that allows any number of clients behind NAT gateways to communicate with a server behind a separate NAT with no port forwarding and no DMZ setup on any routers in order to directly communicate with each other. The server does not need to know anything about the clients trying to connect, nor does it need to communicate with any other hosts in order to initiate the communication. Simply put, this is a proxy server that works behind a NAT, even when the client is behind a NAT. There is no middle man, no proxy, no third party, no UPnP required, no spoofing, and no DNS tricks. More importantly, the client can then connect to any host or port on any remote host or to a fixed host and port decided by the server.