If you like what they have to say, then you can build your own
> sniffer with this program. Just download the version which goes
> with your operating system:
>
formatting link
> We tried it here on Patrick's computer network and it is sitting here
> right now sniffing at his weather station stuff and some email on > another computer.
It is fine to run a sniffer on your own lan, but larger systems, like university lans, usually consider a promiscuous ethernet port, which is key to sniffing, as network abuse. You are likely to get kicked off, or worse.
For diagnostic sniffing, using Linux, I am partial to Ethereal, which is mostly a graphic front end to tcpdump. It can be set to trace specific protocols, which gives it very similar capabilities, although not as user friendly.