Here's a simple hint on an easy way to create attractive abstract images for personal use (for the Desktop, Photoshop backgrounds, on iCards, etc.) with iTunes:
Turn visual effects on in iTunes, preferably full screen
Capture the entire screen with Command-Shift-3. The captured image is saved in PDF format on the desktop.
Using Preview export the image to a known graphic format (e.g., JPG).
Place the graphic in the desired application
[robg adds: Yes, this is a basic hint, but it can be a timesaver, especially if you're trying to generate some interesting patterns for backgrounds or textures. I was able to make a fairly interesting texture by applying a number of filters to a captured snippet of iTunes' visuals.]

Hiding from nmap


Following a story on slashdot led to this article about hiding your system fingerprint from nmap. Now as far as I know (from nmap scanning on my local network), my machine has never been tagged (identified), but the suggestion about dropping packets to closed ports seemed like a "Good Idea." The instructions, in the BSD section were:
sysctl -w net.inet.tcp.blackhole=[0 | 1 | 2]
sysctl -w net.inet.udp.blackhole=[0 | 1]

I used the values (2,1), and nmap was then unable to even propose a set of values (it failed on tests 2,5,6,7 and U). Note sure of the real value but for the paranoid out there, why not give yourself a little extra edge.