Here's a list of some software I find essential and which I usually install on new Windows machines. All are free and most are open source.
Browser: Firefox or Chrome
https://www.mozilla.org/firefox/
https://www.google.com.au/chrome/
Email: Thunderbird
https://www.mozilla.org/thunderbird/
Anti-virus software: Microsoft Security Essentials
http://windows.microsoft.com/en-au/windows/security-essentials-download
Distributed Version Control System: Git for Windows
http://git-scm.com/download/win
SVN Client: Tortoise SVN client
http://tortoisesvn.net/
Flash: Adobe Flash Player
http://get.adobe.com/flashplayer/
File archive/compression tool: 7-Zip
http://www.7-zip.org/
Media player: VLC Media Player
http://www.videolan.org/vlc/index.html
Text Editor: Notepad++
http://notepad-plus-plus.org/
PDF creation: PDF Creator
http://www.pdfforge.org/pdfcreator
PDF viewer: Sumatra PDF
http://blog.kowalczyk.info/software/sumatrapdf/free-pdf-reader.html
Office software (spreadsheet, word processor, slideshow): Libre Office, Open Office or Microsoft Office
http://www.libreoffice.org/
http://openoffice.apache.org/downloads.html
http://office.microsoft.com/
CD/DVD Burner & Ripper: ImgBurn
http://www.imgburn.com/index.php?act=download
Bitmap Graphics editor: Paint.NET or GIMP
http://www.dotpdn.com/downloads/pdn.html
http://www.gimp.org/
Vector graphics editor: Inkscape
http://www.inkscape.org/
Digital darkroom software: LightZone
http://lightzoneproject.org/
Password safe: KeePass
http://sourceforge.net/projects/keepass/
File synchronisation: Dir Sync Pro
http://www.dirsyncpro.org/
Cross posted from
http://www.dodgycoder.net/2014/08/essential-free-windows-software.html
