Be sure to check here -> http://osswin.sourceforge.net/ and http://shsc.info/UsefulWindowsSoftware for up to date stuff.

Free Software Portal on Wikipedia

File Mananagement 

7-Zip - An is a file archiver with a high compression ratio.

InfraRecorder - CD/DVD burning tools

Virtual CloneDrive - a tool to mount ISO images

FreeCommander - file manager

TrueCrypt - encrypted drive

Programming 

XML Notepad - Microsoft's 2008 XML editor

Visual Studio Express - Microsoft's free version of the .NET developer tools

Notepad+ - A great text editor for programmers (with FTP edit)

Python - the python programming language

PyGTK - python GTK bindings

TortoiseSVN - is a really easy to use Revision control / version control / source control software for Windows

Media (Audio and Video)

MPlayer - A DVD/video player

VLC - another video / stream player 

Illiminable - Directshow Filters for Ogg Vorbis, Speex, Theora and FLAC

Codecs - lots of em 

Audacity - audio authoring / editing 


Utilities

Clamwin - open source antivirus

Process Explorer - Microsoft acquired SysInternals this thing is so good

WinDirStat - a graphical look at your disk usage

UBCD - Ultimate Boot CD for Windows, speaks for itself

EasyBCD - Windows Vista bootloader modification tool

GNU Privacy Guard - open source PKI software

Internet

PuTTY - and excellent SSH, Telnet, rlogin, serial terminal client

WinSCP - a excellent SCP/FTP/SFTP client

Pidgin - an instant messenger / IRC client

Mozilla Firefox - the excellent open source. powerful HTTP client (Web browser)

uTorrent - wee bit of fun

Productivity

OpenOffice - free word processor, spreadsheet, database, presentation and MS office compatible

Paint.net - excellent image creator/ editor 

GIMP Windows Installers  - not a big fan of the GIMP but it installs a good GTK+ for Windows

Services

MySQL - open source database server and client tools