There is so much software available for GNU/Linux that it is virtually impossible, and rather impractical, to list it all. Instead, the pages linked below present what I think are the best and most practical software options for each category.
Since I list mostly FLOSS, some of the following software may be already installed on your distros by default. Although I provide links to the official website of each application, usually installation of the software does not require a direct download, but is only a click or command away from installing via your Package Manager.
Web Browsers: You've got lots of options for browsing the web.
BitTorrent clients: Clients for the BitTorrent Peer-to-Peer protocol.
Office Applications: Word Processors, Spreadsheets, Database and Presentation applications.
Audio Software: For composing, editing, organizing and listening to music & podcasts.
Image Viewers & Editors: Image viewers, Photo organizers and Image Creation & Editing.
Games: Gaming are now exploding on GNU/Linux. Come find old and new favorites here.
Emulators: Emulate different or older PC and gaming platforms. Also includes virtualizers.
Back Up Utilities: Use these applications to back up your data.