Other free and open source software
As with the free and open source software for translation, these packages are also available for other platforms, such as Mac OSX® and Windows®.
I have also used each of the recommended packages.
If you need an application for editor for recording, slicing, and mixing audio, you can’t go far wrong with Audacity. Audacity can record live audio, convert tapes and records into digital recordings or CDs, edit Ogg Vorbis, MP3, WAV or AIFF sound files, cut, copy, splice or mix sounds together, change the speed or pitch of a recording and much more.
Calibre is a powerful and easy to use e-book manager. Calibre supports organising existing e-books into virtual libraries, displaying, editing, creating and converting e-books, as well as syncing e-books with a variety of e-readers.
VLC is a great media player that plays anything: files, discs, webcams, devices and streams. It also plays most codecs with no codec packs needed: MPEG-2, DivX, H.264, MKV, WebM, WMV, MP3, etc., and is cross-platform. If you’re happy to use a media player minus a GUI and control it via the keyboard, there’s always mplayer.
If you have an extensive collection of audio files, you might like Kid3. If you want to tag multiple MP3, Ogg/Vorbis, FLAC, MPC, MP4/AAC, MP2, Speex, TrueAudio, WavPack, WMA, WAV and AIFF files (e.g. full albums) easily without typing the same information again and again and have control over both ID3v1 and ID3v2 tags, then Kid3 is the program you are looking for.
While social media may be the flavour of the month, there’s still a role for good old IRC. Xchat is a brilliant dedicated IRC client.
Blender is your tool for 3D content creation.
Desktop publishing/document production
Scribus is package that provides professional page layout with a combination of press-ready output and new approaches to page design.
FTP may be an old protocol, but still has its uses, especially if you administer a website, so don’t forget to add FileZilla to your open source applications list.
For a wider look at free and open source software recommendations, see this list on Mashable.