I was looking for a replacement for Power Archiver (the once best archiver for windows), since they have now required licensing after 31 days (as does WinZip for Windows). After digging around, I found a slick little archiver app called 7-Zip, which supports all the major archive formats, written open source, under the LGPL license and has a clean interface.
Also, the recent release of the open source graphics program, Gimp 2.2 for Windows, makes Adobe Photoshop obsolete!! :-)
Gaim for Windows replaces Trillian and handles multiple accounts without merging lists – like trillian does :-(
FileZilla is the best gui ftp program that is open source for windows operating systems.
Of course, FireFox is the best browser out there, also open source and ported to Windows – checkout the Adblock toolbar.
Thus the last remaining reason to keep a windows computer around is Quicken, still the best of the best financial management software.