For a piece of FLOSS (free/libre and open source software) to be here, it has to both be personally useful and I have to like it a lot:

backup: rdiff-backup

desktop: GNOME

development: Python, git-cola

misc utilities: grep, Sudo, reprepro, Lsof, less, Meld, Uptimed, top, Tracker, Ex Falso, wajig, GNOME system monitor applet, xfrun4, coreutils (wc, tail), LinkChecker, Iotop, man, OpenSSH

spreadsheet: gnumeric

terminal emulator: GNOME Terminal

virtualisation: VirtualBox

version control system: Mercurial

web browser: Chrome/Chromium

non-FLOSS: Disqus, Gmail, Stack Exchange, Bitbucket