OpenOffice-Openoffice is substitute to Microsoft office.It is open source and cross-platform.
Gimp/GimpShop– A substitute to Adobe Photoshop which is an image editing tool.
OpenLazio– An Adobe Flash like software that supports .swf files.
Avast/Avg-Avast and Avg are good & free alternativates to paid Norton Antivirus,Mcafee etc.
Itunes: Free audio player from apple that supports mp3,wav,wma.
QuickTime-Free video player from apple that supports .mov.
Klite-Codec Pack– The free Audio/Video Codec Pack that allows you to play almost all files.
Firefox-The undoubtally the best browser which is free.(alternative to Internet Explorer.)
Opera-The browser which is rival of Firefox.
Mozilla Thunderbird/Eudora– An alternative to MS Outlook.
Mysql-A free alternative to manage database, replacement to suites like Oracle.
IrfanView-A free Image Managing Tool.
7Zip/PeaZip-A free Zip/UnZip Utility.
Ranish Partition Manager/GParted-Free Partition Manager
IrfanRecorder/CDRao-Free CD/DVD Burning Software.