Command-Line Interface, i.e., a console/Terminal tool.
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 curses front-end for it.