Here are some interesting tools for people who like to stay in the terminal as much as possible:
TIG (Text-mode Interface for Git)
This tool lets you navigate your Git logs with Vim keybindings. I use this instead of git log. Previously, I had aliased gl to git log, but now it loads tig.
(Magit looks very interesting too, but it runs in Emacs, not the terminal.)
Ranger File Manager
This is a command line file manager with Vim keybindings.
(Side note: another useful Git tool for visually browsing local repos is Git Cola.)
Lynx Browser
This is a text browser which can be useful for some things. I use it when I show up at a wireless hotspot that has a splash login screen. If I launch my regular browsers, all of my open tabs (from the previous session) will load the splash page if I’m not authenticated with the hotspot. If I launch Lynx, then I can log in to the hotspot before opening my regular browsers.
Lynx also makes a good text scraper for quick tasks, especially if you want to get a list of links on a webpage.
Example usage as a scraper:
$ lynx --dump https://codeselfstudy.com/ > scraped_webpage.txt
Irssi for IRC
For IRC: https://irssi.org/
Vim or Neovim
A nice text editor. 
See the Vim tips thread.
Other Tools
Does anyone have other interesting command line tools to recommend? I’ll probably add a few more in the comments below later.
Edit: see also CLI: improved (command line tools)