Well.......
Linux in all its flavors, as well as the BSD's - each has a purpose for me, and I use them regularly.
KDE - specifically KDE 4, is an amazing desktop environment, the best I've come across, imho, with the potential to go so much further. If you haven't tried it..... do it. Do it now.
Amarok - makes iTunes look like it was coded by children.
KOffice & OpenOffice - maybe I should just say "anything that uses the Open Document Format (ODF)".
LiVES/Cinellera/etc - for my digital video editing needs.
JACK/Ardour/etc - for all my digital audio workstation needs
Are you sure you want the rest of my list?