First of all, there's a piece of software built upon GNU/Linux and Debian called Ubuntu. Ubuntu is the only operating system I have installed on my PC and I do not need anything else. No dual-booting with Windows. My old useless Windows XP edition on my second SSD had long made space for pictures and videos of my family and my little baby boy.
From my shiny Linux PC I regularly start a variety of free, open-source programs.
- Firefox - I guess you know that one, from which I am just writing these lines. I love it and it gets better everyday.
- Inkscape - Helps me with ANY "arrows and boxes" and (scientific) line drawing tasks, esp. for my Job. I use it instead of PowerPoint, since it is much easier to use and much more powerful. It never is in my way, and never tries to be smarter than me, which I think is critical for creativity software. At home I also use it to design greeting cards or stitching templates for my mother.
- Gimp - A powerful image manipulation tool, that helps me retouch fotos and create art, such as the following Undying retouch, Supernova image, and fine Pixel Art:
PS: Gimp 2.8 just got better with single window mode!
- LyX/LaTeX - If you ever consider writing a scientific document (paper, thesis, etc.) use LyX! LyX uses LaTeX which can make your document look very professional and LyX frees you from all the technical details that you need consider when using plain LaTeX.
- Eclipse - This is a really big one. Even though I prefer to write code in Vim, there a some cases where a more integrated tooling can be useful. I use it mainly for writing programs for the Java VM.
- Wine - This is a two-edged sword. On the one hand, Wine is free, open-source software that I have been using since the day I started to use Linux in 2006. On the other hand, Wine's only purpose is to run mostly, non-free Windows-based software under Unix/Linux/MacOS. I use it mainly for gaming. It helps me to run very old Windows Games, such as Fallout 1 that may not even run out of the box on Windows itself anymore. I also use it to run very recent games, such as Diablo III, or Drakensang II, or Torchlight.
- Gnome - My Desktop of choice. Yes, in GNU/Linux you are free to chose from a variety of Desktops.
- Meld - A really nice tool to compare files.
- Git - A modern, light weight, version control system. I also use it to backup my data to/from file shares and USB drives.
- SumatraPDF - Very fast and lightweight PDF viewer. Much faster than the bloated Adobe Reader.
- Mawk, Screen, and similar Core Utils - Perfect tiny tools that ease my everyday work in the office.
- Freemind/Freeplane - Mindmapping tools that help me organize my ideas and thoughts.
- Stellarium - Nice, ejoyable night sky and universe explorer
- Andor's Trail - Very addicting, open-source RPG for Android
- Battle for Wesnoth - Awesome, extensive, community-driven turn-based strategy game
That's it about free, open-source software that I love nearly as much as I love my wife, my little baby boy, and my cats. ;)