Though my preference is Linux, many of the Open Source Alternatives listed here are also available for Windows and Mac. Open Source Windows  and Open Source Mac are great short cut sites for download links if you wanna try out these packages and many others! For Linux, it is much simpler as most major distro's have these packages in their repositories and will install via a single mouse click or one line command

Commercial Offering Open Source Alternative Comments:
Microsoft Windows Linux
Duh...do you have to ask?!?
Microsoft Office Open Office *
Adobe Photoshop The GIMP *
Microsoft Explorer Mozilla Firefox *
Microsoft Outlook Mozilla Thunderbird *
Microsoft SQL MySQL *
Microsoft Picture Manager F-Spot *
Microsoft Visio Dia *
Roxio RecordNow K3B *
Adobe Premier Kino *
Adobe Illustrator Inkscape *
Adobe Framemaker Scribus *
MSN Messenger Pidgin *
Microsoft Media Player Amarok *
CyberLink Power DVD VLC Media Player *
WinZip 7-Zip *
Microsoft Frontpage Kompozer *
SlickEdit Geany *