Waaaay back when Spirit and Opportunity landed on Mars in January 2004, the Planetary Society helped the public participate in the missions with a number of projects, including one where we printed "secret codes" around the edges of the two names-bearing DVDs that were bolted to the Mars Exploration Rover landers. The secret codes weren't revealed until the rovers themselves took photos of the DVDs and returned them to Earth, and then people got to decipher the messages for prizes. Not quite two years later, we redesigned our website, and one of the pages that didn't get ported over to the new design was the page that gave people clues for deciphering the codes.
Nobody missed it until this year, but suddenly I've had a slew of inquiries about clues to deciphering Spirit's DVD. So I dug into our old website backup, and have now reposted the clues to decoding Spirit's DVD. Even if you've solved it before, if your memory is as bad as mine is, you may enjoy solving it again!
NASA / JPL-Caltech / Cornell
Spirit Sol 2 DVD photo: L4 (red) filter
On Sol 2, the rover's second Martian day, mission engineers commanded the Panoramic Camera (Pancam) instrument to capture four images of the DVD assembly. The four images were captured through four different filters: a red filter, a green filter, a blue filter, and an "empty" filter. This is the "L4" or red filter image.