Networking/Diagnostics
freeware
Bopup Scanner
Bopup Scanner is a freeware network scanner that displays active computers with logged user names (NetBIOS), MAC and IP addresses. Bopup Scanner also recognizes and shows HTTP (Web) servers running on remote computers (TCP ports 80, 8080), quickly detects online computers, allows to browse shared resources of remote computer and save results to a text file. Advanced users can enter IP range to scan, specify a timeout to resolve a remote host and run the program from a command line interface.
open source
Wireshark
Wireshark (formerly Ethereal) is a network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet. Has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session.