OFFICE SOFTWARE
Microsoft Office patches and addons
The Microsoft Office Compatibility Pack (offered by Microsoft) provides the ability to open, read, edit and save Word/Excel 2007 files with Word/Excel 2000 and 2003.
Other applications
Network Notepad is a freeware application for drawing network diagrams; it can also be used to create flowcharts.
Microsoft has an excellent application for stitching together images called Image Composite Editor, which requires Microsoft Silverlight.
SECURITY SOFTWARE
Antivirus tools
Microsoft Security Essentials for Windows XP / Vista / 7
Antispyware tools
Spybot Search & Destroy (there is a portable version as well)
Anti-rootkit tools
Sophos Anti-Rootkit is a tool for detecting and removing root kits from Windows XP, Vista and 7.
Virus removal tools
AVERT Stringer is a free virus removal tool from McAfee.
ClamWin is an open source virus scanning tool that is very effective in finding and eradicating viruses, but it does not have any autoscan features. (Hey, you get what you pay for ...) There is also a portable version for running from a USB stick.
VIPRE Rescue is a virus removal tool that is run from the command line after booting Windows in safe mode. This tool is specifically intended for using on systems that are so infected that you can't run the O/S to do any normal operations.
EDITORS
IDEs
Bloodshed Dev C++ is a C/C++ IDE for Windows that uses a MingW port of GCC
Text editors
Acrosoft's AcroEdit is a "totally free" text editor with column editing, syntax highlighting, built-in FTP, a diff utility, and more
XML
The Essential XML Editor is an open source XML editor for Windows
GRAPHICS
Easy Thumbnails is a freeware app for Windows that can be used to easily and accurately scale down image files
UTILITIES
Backup tools
FreeFileSync is an open source application for synchronizing files between drives, much like Microsoft's Sync Toy.
Command line tools
FTPDMIN is a very simple FTP server (written by Matthias Wandel) designed to be run temporarily on a Windows system. Very handy for quickly transferring data between systems, but no login controls or other security provided.
A number of Linux tools have been ported for use with the Windows CLI (Command Line Interface) by GnuWin:
Database tools
Reportizer is "band oriented database reporting tool"
File compression/decompression tools
ZIP and UNZIP (from Info-Zip) are open source command line tools for creating, updating and unpacking ZIP files.
7-Zip is a free GUI tool for dealing with ZIP files; a 32-bit version and a 64-bit version are offered.
Linux connectivity tools
Ext2Read is a GUI tool that allows browsing a Linux partition from a Windows system (useful when setting up a dual boot Windows/Linux system)
Xming is an X window server that can be run on a Windows system in conjunction with PuTTY so that the Windows system can be an X window client to a Linux system. I was able to quickly set up a Windows Xming/PuTTY session by following a good PuTTY-Xming tutorial..
Low level tools
SysInternals tools are now owned by Microsoft, but a pre-Microsoft set is still available in the form another user originally downloaded them and as he found them on BitTorrent
UltraDefrag is an open source tool for defragmenting hard drives on systems running Windows NT 4.0 and later
PDF tools
BullZIP PDF printer, a freeware print driver that creates a virtual printer that can be used to create PDF files from any Windows application that can print
Performance Analysers
Fresh Diagnose can be used to benchmark a system's performance
CD/DVD TOOLS
Infrarecorder is a free CD/DVD burning tool for Windows
IM TOOLS