Originally Posted by Salem Network analyser - http://www.ethereal.com/
Use this to
1. Watch what existing browser / email / ftp / chat client does already
2. Watch what your program is doing different, if you're writing a browser / email / etc
Ethereal is DEAD. :(
Well, not really... The name has changed, it's now called WireShark: http://www.wireshark.org/
Arduino - HomePage - very cool. It's a predesigned I/O board that you can load C programs onto.