Geektool takes shell commands and lets OSX geeks overlay their output on the desktop. (It does more than that, actually… it’s really quite awesome.)
Remind is a powerful, command-line based reminder app and calendar generator (helpful wiki).
I’ve been pushing on Snooze lately, our imminently-releasable persistence layer for PLT Scheme. I thought to myself: “How quickly could I knock up a GTD DB using Snooze?”
Turns out, not more than an hour or so to get something rolling. Now, I have a simple command-line GTD interface that uses Snooze for persistence, remind for rendering of content, and Geektool for rendering things to my desktop.
The lower-left portion of my desktop has a six week calendar overlaid, and above is an eight week calendar with a condensed list of the same information.
I’ll publish the GTD interface when I’ve had a chance to work with it for a while. For now, I suspect it will evolve as I decide how I like working with it best.