Yes, I'm aware of the irony of a publication called PCWorld covering a piece of Mac software, but this one was too fun to resist. Cathode is a new app that lets you access Mac OS X's Unix-based command line.
While that in itself is nothing special--OS X has always shipped with a Terminal app--Cathode simulates the experience of using a command line on an old green CRT.
The app itself is $20, but you can try it out for free...with a catch. If unless you get a license, the display quality will degrade as you use it, much like an old CRT screen fritzes out as it ages. (If you pay the license fee, you can adjust the degree to which the quality degrades.
OK, so it's a bit of a gimmick, and whether you think it's worth the $20 (£12.50) is up to you, but it's one of the nerdiest apps we've seen recently. If you're interested, go ahead and give it a try.