Network

AngryIP (free)
http://www.angryziber.com/

The Network Toolkit (2 Versions, free and full)
http://www.cacetech.com/products/toolkit.htm

Networking Tools (free)
http://roadkil.net/listing.php?Category=4

Network Supervisor
Admin's Port Scan
...
http://www.freeware.de/autoren/Autor_Hoffmann_Timo_1874.html

Ethereal, Wireshark (os/free)
http://www.ethereal.com/
http://www.wireshark.org/

Cain and Abel (free, be careful!)
http://www.oxid.it/
Can be used for illegal purposes, which is forbidden!

os: open-source; free: freeware [mostly for non-commercial use];
trial: evaluation version, mostly limited to 30 days;
share: shareware [1. free to use with all features for a short period of time, or 2. free to use with limited features - or nag-screens(winzip)], both kinds have to be paid to become the full versions