LAN monitoring and diagnosing tools

Ethereal: Full featured, runs on windows, does 802.11
http://www.ethereal.com/

Nmap: scans whole network, lists hosts and open ports,
http://www.insecure.org/nmap/index.html

More for security concerns, but looks powerful
http://ettercap.sourceforge.net/

Stats on current network activity
http://cebu.mozcom.com/riker/iptraf