Command-Line Interface, i.e., a console/Terminal tool.
Essentials
I’ve been using some of this stuff for over a decade now (or their predecessors), and they tend to stick around.
- speedometer, a neat Python curses-based monitor.
 - nc - the Swiss Army Knife of any WAP tester worth his salt.
 - tcpflow - plaintext dumps of HTTP traffic without the hassle of parsing tcpdump output.
 - vim - the editor. Accept no substitutes.
 - SSH - you do want to use all of these securely, now don’t you?
 - wget - Oh, how can I request thee - let me count the ways…
 - unison, which has mostly replaced my old set of sync tools:
 - screen - virtual desktops, VT100 style
 - procmail - keep your mail tidy
 - oleo, a text-based spreadsheet.
 - links, a text-based browser.
 - remind, a text-based calendar and alarm program (via 43 Folders)
- Wyrd, a 
cursesfront-end for it. 
 - Wyrd, a