Top Free Software

First, the absolute essentials:
Windows Updates (http://windowsupdate.microsoft.com)
Flash [browser plugin] (http://www.adobe.com)
Shockwave [browser plugin] (http://www.adobe.com)
Reader [PDF file viewer] (http://www.adobe.com)
Java [enable playing of java apps] (http://www.java.com)
Quicktime [media player] (http://www.apple.com/quicktime/download)
Security Essentials [anti-virus] (http://www.microsoft.com/Security_Essentials)

Next come the very Useful:
CCleaner [system cleanup] (http://www.ccleaner.com/download)
CutePDF [PDF writer] (http://www.cutepdf.com/Products/CutePDF/writer.asp)
VLC [media player] (http://www.videolan.org/vlc)
Open Office [Office Suite] (http://www.openoffice.org)
Gimp [photo editor] (http://www.gimp.org/downloads)
Irfanview [graphic viewer] (http://www.irfanview.com)
FreeCalc [calculator] (http://www.moffsoft.com/freecalc.htm)
Windirstat [directory statistics] (http://windirstat.info)

And then there are the misc:
IESpell [Add spell check to IE] (http://www.iespell.com)
Stellarium [planetarium] (http://www.stellarium.org)
Wallmaster [manage desktop wallpaper] (http://www.tropicalwares.com/wallmast.html)
Filezilla [FTP Client] (http://filezilla-project.org/download.php?type=client)
CRON-o-meter [diet tracking software] (http://spaz.ca/cronometer)
Google Earth [virtual globe] (http://earth.google.com)
Windows Live Essentials [Misc Collection] (http://download.live.com)
Audacity [Sound Editor] (http://audacity.sourceforge.net)

Finally, the alternatives*:
Google Docs [Office Suite] (http://docs.google.com)
Firefox [web browser] (http://www.mozilla.com/en-US/firefox/ie.html)
Chrome [web browser] (http://www.google.com/chrome)
Safari [web browser] (http://www.apple.com/safari)
AVG [anti-virus] (http://free.grisoft.com)
Avast! [anti-virus] (http://www.avast.com/free-antivirus-download)
PCTools [anti-virus] (http://www.pctools.com/free-antivirus)
Avira [anti-virus] (http://www.free-av.com)
Clamwin [anti-virus] (http://www.clamwin.com)
ZoneAlarm [firewall] (http://www.zonealarm.com/security/en-us/zonealarm-pc-security-free-firewall.htm)
Comodo [Firewall] (http://personalfirewall.comodo.com/free-download.html)
* These are listed as alternatives either because Windows already comes with something that does this, or because I have listed an item above that performs the same function.

Disclaimer: I didn’t have to pay for anything, but I have made voluntary contributions to many of these projects in the past.

This list is not meant to be all inclusive, so here are some other lists of free software:
http://www.pcmag.com/article2/0,2817,2338803,00.asp
http://100-downloads.com
http://www.opensourcewindows.org
http://lifehacker.biz/articles/best-open-source-software
http://downloadpedia.org/Open_Source_Windows