Powertoys:
Cmd Here
Task Switch
TweakUI
Third party apps:
TXMouse
VirtuaWin
DAEMON Tools
Skype
Privoxy
Java
Flash
Replacements:
Internet Explorer -> Firefox, Opera
Word -> Abiword
Office -> OpenOffice
Outlook -> Thunderbird/Sunbird (unless using shared office calendar and address book)
Windows Media Player -> Media Player Classic, VLC, XP Codec Pack
Windows Messenger -> Gaim, Psi
Paint, Visio -> Inkscape, Dia, GIMP
Acrobat Reader -> Ghostscript & GSview
Notepad/Wordpad -> GVim
WinZip -> 7-Zip
GnuPG -> WinPT
Unixy additions:
MinGW/MSYS
Cygwin
GNUWin32
coLinux/andLinux
Strawberry Perl
PuTTY
WinSCP
Python
Unix interop:
Services for Unix (free from MS)
Xming (X.Org X server)
Cygwin (includes an X server)
mingw32/MSYS (Unix style build environment for Windows)
True X-Mouse (Unix style mouse focus and highlight-copy semantics)
VirtuaWin (Pseudo-virtual desktops)
WinSCP (SCP/SFTP client)
PuTTY (SSH client)
GNU-Win32 (Win32 ports of Unix utilities)
Strawberry Perl (Win32 “Official” Perl port)
coLinux (Linux as Win32 process)
Kerberos for Windows
OpenAFS client (to access AFS fileservers)
LyX (LaTeX technical writing GUI)
TightVNC (VNC client and server for platform independent remote framebuffer access)
Simplification:
gVim (text editor)
AbiWord/OpenOffice (office suites)
Sunbird (Calendar with WebDAV support)
Firefox/Opera (browsers)
ImageMagick (image conversion and viewers)
GIMP (image editing)
GTK for Windows (for Dia/GIMP/etc)
Inkscape/Dia (chart drawing)
Media Player Classic and XP Codec Pack (media player)
7-Zip (file archiver)
Ghostscript/GSview, Foxit Reader (PDF/PS viewer)
Powertoys (enhanced Alt-tab, command line here, TweakUI)
Icon Restore
Psi (Jabber IM client)
Gaim (Multi protocol IM client)
Process Explorer (Task manager replacement)
DAEMON Tools (virtual CD/DVD driver)
Java 2 SE 1.5 or greater (replace Microsoft JVM)
Privoxy (filtering HTTP/HTTPS proxy server)
TortoiseCVS/TortoiseSVN (shell integration for revision control systems)