[Registered users of my site can access a whole series of articles and tutorials on security and networking tools, including BackTrack. Here’s a taste.
hping, hping2 and hping3
The various versions of hping provide either a command-line or TCL interface. All of them are used to craft packets at the very lowest level: you can choose the network protocol and individual packet flags, spoof your addresses, flood a target, pop a shell and transfer files.
This is a phenomenally powerful tool, one that can do simple, stupid DOS attacks, or brilliant, subtle exploits; take your pick. There are a whole lot of good tutorials and how-tos on the Internet, so I’m linking you to some of these.
Read this one first at The Ethical Hacker Network: http://ethicalhacker.net/content/view/72/24
Some good examples at Linux-Magazine.com: http://www.linux-magazine.com/Issues/2009/99/Hping/(offset)/6
Very short examples: http://rationallyparanoid.com/articles/hping.html
One lengthy procedure: http://www.compuhowto.com/linux/hping3-examples/
A 5-part tutorial at TheTazZone.com: http://www.thetazzone.com/tutorial-hping-basic-host-and-port-probing-tut-1-of-5/