The Wahooga list of essential software for Technical Support people.
Windows
VMWare Workstation
Sure you could use Microsoft Virtual PC 2007 for free but with VMWare Workstation you can use snapshots which are ideal for working on customer problems and restoring to a point in time. Also provides better platform support. [commercial]
Port Reporter
Port Reporter a logging service for Windows that logs TCP/IP port usage data. [freeware]
Gadwin Printscreen
This screen capture software is always one of the first bits of software I install on a new PC. Its great for producing documentation and tutorials or just saving errors and screenshots for later reference [freeware]
Sysinternals
Phenomenal utilities to help you manage, troubleshoot and diagnose your Windows systems and applications [freeware]
SpaceMonger v1.x
The last free version of SpaceMonger a tool for keeping track of the free space on
your computer. [freeware]
ToDoList
Very useable ToDo list management software. [freeware]
Internet
WebScarab
Java app for analysing applications that communicate using the HTTP and HTTPS protocols. [freeware]
Fiddler
Fiddler allows you to
inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. [freeware]
Developer
Reflector
Class browser, explorer, analyzer and documentation viewer for .NET with many plugins available [freeware]