Linux has withstood the test of time and gathered an interesting repertoire of software over the years. This is a list that moves from my most used to my least used software on my system. Some of the tools here are widely used and advanced, but if you aren't already using them they're worth a gander.
Git is the version control programmed by Linus Torvalds, the same guy that wrote the Linux kernel. Git allows a user to keep snapshots of changes of a project on their computer. git is SCM and not very useful for binary data, but it works wonders for projects that need collaboration or just a very secure way to roll back changes. Git will use all available cores for compression, making it the fastest SCM in existence. Git is supported by websites like Google Code and Github.
Vim is Vi-Imporoved. For readers who are unaware, vi is the standard Unix editor. If you have a Linux system, you have vi (this is even true for Android). Vim improves on vi extensively, adding syntax highlighting, improved navigation, and extensive plugin support. Even though graphically I prefer emacs, for remote sessions or quick edits nothing beats Vim. What sets vim apart from Emacs is modal editing, where the user has two modes to edit the file in.
Last but not least we have mplayer. mplayer (and mplayer2) are tools for the command line that can play video and music of an incredible amount of formats. mplayer will use X to draw a window for a video, but it can play video in text completely with ASCII animations.