Normally I wouldn't post this, since there's quite a few of these laying around here, but this website is quite unique. It has a huge amount of network toos, which makes at a decent bookmark to anyone. For those of you looking for older versions of some tools, this one contains tools dating back to 1996(i doubt the links will even work). Have fun playing around with these

http://www.slac.stanford.edu/xorg/nmtf/nmtf-tools.html