Essential Network Tools

Putty
An open source telnet and SSH Client for the Windows and Unix platforms.
http://www.chiark.greenend.org.uk/~sgtatham/putty/

Putty Connection Manager
PuTTY Connection Manager is a windows PuTTY tool to manage multiple PuTTY instances using tabs in a single window.
http://puttycm.free.fr/

Wireshark

Wireshark is a network protocol analyzer for Unix and Windows
http://www.wireshark.org/

Network Scanner
Network Scanner is a free multi-threaded IP, NetBIOS and SNMP scanner with a modern interface and several advanced features.
http://www.softperfect.com/products/networkscanner/

LAN Search Pro
LAN Search Pro is a free utility for fast file searching across your LAN. You can search for specific file names or for specific file types (e.g. all MP3 files).
http://www.softperfect.com/products/lspro/

Solarwinds TFTP Server / Client
SolarWinds free TFTP Server is a multi-threaded TFTP server commonly used to upload and download executable images and backup configurations for routers and switches.
http://www.solarwinds.com/products/freetools/free_tftp_server.aspx

RealVNC
RealVNC provides remote control software which lets you see and interact with desktop applications across any network.
http://www.realvnc.com/products/free/4.1/index.html

WinSCP
Open source freeware SFTP, FTP and SCP client for Windows.
http://winscp.net/