Linux Open Source Software for Mac OS-X

If you run Linux and install free software, you won’t have to worry about this ever again! Most of free (as in free speech) software is free (as in free beer). You can find a free replacement for most of the commercial software out there. They might lack some of the advanced functionality, but they’ll be more than enough for most people. -- Linux and Open Source Blog

Most of these are available for Mac OS-X as well. I use Firefox, Thunderbird, AdiumX (port of GAIM), NeoOffice (OpenOffice port), VLC and The Gimp regularly.