I have used open source applications for some years now, and prefer them not only because they are free and often of very good quality, but also for how they are created and the philosophy behind them. My most recent switch is from InDesign to Scribus for layout type work.
Other applications I find especially helpful: Firefox browser, VLC Media Player, Inkscape (vector design) and sometimes Gimp (image editing), WordPress (including for this blog) and other content management systems, FileZilla for ftp (file transfer), and also Celestia (3D space simulation). I have also started to explore Blender which is 3D animation software.
I used Open Office for a while, but have now switched to Star Office which is part of the Google Pack. (See below.)
And some that are free and good quality, although not necessarily Open Source: Google (gmail, calendar, reader, documents, photos), Google Pack (Picasa, Google Earth, Star Office, SketchUp), Skype (online calling) and Stellarium (planetarium).