Obviously, OpenOffice.org and Firefox. Thunderbird for e-mail. SeaMonkey. Opera isn't open source but it's free as in beer and cross platform. It works the same in Windows and Linux and can even use the same e-mail files and many of the others (address book, notes, bookmarks, to name a few).
The Gimp has a Windows version. So does Scribus. AbiWord has a Windows version. XnView--I'm not sure it's open source. I *think* Inkscape has a Windows version. Audacity.
I haven't used most of these in Windows, just Linux. However, if we're talking about transitions to Linux, switching to these programs under Windows is a help because they are just about the same in Linux.