cryptcat for Linux
Cryptcat is the standard netcat enhanced with twofish encryption.
Ports for WIndows NT and Linux exist at http://www.farm9.com
Twofish is courtesy of counterpane, and cryptix. We started with the Java version of twofish from cryptix, converted it to C++ (don't ask why), and enhanced it by adding CBC mode and the ciphertext stealing technique from Applied Cryptography (pg. 196).