Network diagnostic tools? Sniffer software?

Anyone out there who works in networking support have a great (preferably free) tool for diagnosing sporadic network issues?

What I really would like is some small program that we can install on a computer to monitor all the network traffic and keep statistics.

I would like to be able to test and compare bandwidth between different locations as well as monitor it over time.

any suggestions? Thanks.

Umm, it helps to know what OS you’re planning on monitoring.
I only manage unix servers so the ones I can recommend are mrtg, ntop, and big brother. There are plenty of freeware network utils out there at least for unix. If you are stuck with a windows platform though, all is not lost. Simply get cygwin with gcc and you can build many of the GNU programs. It’s just a wee bit more work.

Sorry, it is a windows network. Windows 2000 servers, and all sorts of windows clients from win 95 on up.