Libre Office - full suite of office software, including excellent apps for text documents, spreadsheets, presentations, and databases.
GIMP - Powerful and flexible, open source image editor GIMP is as close to Adobe Photoshop as you can get without opening your wallet.
VLC Media Player - world’s most popular free media players, and for good reason – it can handle just about any audio file, video file, or media stream you can throw at it, without the need to mess around installing additional codecs.
Mozilla Firefox - browser wars show no sign of ending, but Firefox's open source heritage makes it incredibly flexible.