As I hop around between various operating systems all the time (Mac, Windows, Linux, & Solaris) I have found the following applications to be very handy as I don't have to learn a new app for the various OSs. This also could come in handy if you are thinking about switching from Windows to Mac or Linux, if you get use to the follow apps (in place of Windows only apps) it will make your transition a lot easier.

 http://www.freesoftwaremagazine.com/blogs/the_os_agnostic

• Firefox: web browser. OS support includes GNU/Linux, Mac OS X and Windows. Firefox was my first exposure to FOSS and still one of my favorites. Remains popular because it fits the needs for most users.
• Thunderbird: email program. OS support is the same as Firefox.
• OpenOffice: Office suite similar to MS Office. Supported OS's include MS Windows, GNU/Linux, Sun Solaris, Mac OS X, and FreeBSD.
• Gimpshop: image editor. A hack of the GIMP intended to replicate the Adobe Photoshop interface. Usually an easier transition for anyone with exposure to Photoshop, but you can still use the original GIMP. OS support for Gimpshop includes GNU/Linux, Mac OS X and Windows.
• Sunbird: calendar program from Mozilla. Same OS support as Firefox and Thunderbird.
• LyX: document processor. Windows, GNU/Linux and Mac OS X are the supported OS's.
• Scribus: desktop publishing. OS support includes Windows, GNU/Linux, Mac OS X.
• Blender : 3D Graphics. Supported OS's are Windows, GNU/Linux, OSX, Solaris and FreeBSD.
• GAIM : Instant messaging program for GNU/Linux, BSD, Mac OS X, and Windows.
• Azureus: BitTorrent client running on Windows, Linux, and Mac OS X.
• Audacity: Audio editor and recorder for Windows, Mac OS X and GNU/Linux.
• Abiword: Word processing. Supported OS's are Windows, GNU/Linux and Mac OS X.