Linux Bridge - use a Linux box with multiple interfaces like a switch.  A great way to monitor  or shape traffic without changing routing.

Ethernet Taps - There are times that you want to look a traffic on your network (and you aren't already using linux boxes as routers).  Here is a comparision of various methods of tapping into Ethernet for traffic monitoring.

NetCat - The fastest way to send a large file, directory tree, a partition or even a whole HD across the net.

Legal Issues in WarDriving - Its legal to view or record them; illegal to access them where specifically unauthorized.  (Warning, standard disclaimer: my legal advice is worth what you paid for it :) )

Using HPING2 to check Iptables and other firewall rules.
Want to set the "evil bit"? (reserved IP header flag) patches for hping2 and hping3 thanks to Valdis Kletnieks.

Encrypted USB filesystem when you need to keep secure files off your harddrive.