pwnat, pronounced "poe-nat", is a tool that allows any number of clients behind NATs 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.
sbd is a Netcat-clone, designed to be portable and offer strong encryption. It runs on Unix-like operating systems and on Microsoft Win32. sbd features AES-CBC-128 + HMAC-SHA1 encryption (by Christophe Devine), program execution (-e option), choosing source port, continuous reconnection with delay, and some other nice features. sbd supports TCP/IP communication only.
Unix-privesc-checker is a script that runs on Unix systems (tested on Solaris 9, HPUX 11, Various Linuxes, FreeBSD 6.2). It tries to find misconfigurations that could allow local unprivileged users to escalate privileges to other users or to access local apps (e.g. databases).