The most important IMHO, are:
Firefox, and OpenOffice.org
:arrow: Mentioned many times on this forum, but Firefox is the most secure, safest, and fastest browser on the web, it is a much needed replacement for Internet Explorer.
:arrow: OpenOffice.org is an excellent replacement for MS Office. We have been using it for several years in our business for Spreadsheets and Text documents, and it even supports saving documents to PDF! Very useful, and FREE!!
I can't imagine using a computer without these two open-source programs.