For example, I can trace the evolution of my progress by looking at the pure complexity of my desktops over time.
Since many of my pictures are stashed on an old webserver I use, I'll start with those...
beware that some of my wallpapers are... risque?
Back in 2008, I had a really quite sparse layout that packed a lot of information into a small amount of space: http://sonof.bandit.name/files/morestuff/pictures/desk1.PNG
Later on, my desktop became much more mundane, and in 2009 my layout changed styles: http://sonof.bandit.name/files/morestuff/more_pictures/images/qqqqq.JPG
Here's what my machine looked back in my junior/senior year of high school: http://sonof.bandit.name/files/morestuff/pictures/xfce3.png
Once I was out of highschool my layout changed a lot: http://sonof.bandit.name/files/morestuff/more_pictures/images/desktop_2010_01_31.png
My desktops changed to lighter as my programming increased and I headed off to college
http://sonof.bandit.name/files/morestuff/more_pictures/images/scrn-25_Apr_10-180435.png
http://sonof.bandit.name/files/morestuff/more_pictures/images/scrn-10_Jul_10-003337.png
After not cataloging my desktops much for nearly a year, I have noticed that my main desktop today is much, much lighter:
http://i.imgur.com/GitGY.jpg
What's your desktop metamorphised into?