I discovered "man 7 packet" last week and have been having a evil fun filled week disrupting the local network @work.
http://saju.pillai.googlepages.com/pong.c.txt spoof's an ARP packet. If the target of the spoof is a Win32 or WinXP box - interesting things happen - possibly including complete disruption of networking on the target box (I found that out the hard way)
No Malice. Just world peace.