One of netcat's simplest uses, and that which I find most useful, is to create quick and dirty pipes over a network. For example:

Transferring files between two computers, preserving everything:
Recieving box: nc -l -p 65189 | tar xvf -
Sending box: tar cvf - files | nc 192.168.0.4 65189

Capturing from a digital TV tuner on one box, encoding on another, and then sending to a third for storage:
TV box: cat /dev/dtv | nc 192.168.0.3 48172
Encode box: nc -l -p 48172 | mencoder -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=2000 -tsprog 2 -o outpipe - & cat outpipe | nc 192.168.0.4 27184
Storage box: nc -l -p 27184 > tv.avi

Showing resident PHB why switches are better than hubs:
Box 1: cat /dev/random | nc 192.168.0.5 81139
Box 2: nc -l -p 81139 > /dev/null
Box 3: wget http://intranet/index.html

I think you get the idea...