There was once an appearance theme for Mac OS 8 and 9 called Neutech, by Flanksteak Design. I don't think I particular liked the theme as a whole, but it included a desktop background that has been my favourite background for the last eight years or so since I discovered it:
Here's a detail:
I have made sure I that I always have a copy, which has survived across all the different Macs I've used since. However, the image is only 1024 × 768 pixels, which wasn't small at the time, but makes the image look rather pixelated and blurry on modern displays. I have made several attempts in the past to reproduce it in Photoshop, but never got very far. Today, I sat down again with renewed determination and finally found the secret recipe to emulate the look of the original:
One thing that really helped here was Photoshop CS3's Smart Filters, because I could experiment and keep tweaking the many effects I had to apply. This also means that I can easily produce updated versions in the future as screen resolutions increase.
For the time being, here's a 2560 × 1600 version.
Update: I've changed the grid to look more like dried earth and less like reptile skin, and made minor changes to shading.