A few years in the Martian environment can make a rover very dusty. Just ask Curiosity, which recently took a new self-portrait after 613 days spent on Mars. Compare it to an image taken on sol 84, a year and a half ago. It's fascinating to see where dust has accumulated (on the deck, especially in the empty wells where the Mastcams and Chemcams were parked until after landing). It's cool to see how some bits of gravel that were tossed onto the deck during the landing have disappeared, while others have just shifted position slightly. Drag the green slider in the middle of the image back and forth to compare the "before" and "after" states.
A big thanks to Thomas Appéré for mosaicking and aligning those images for me, and to Brandon Schoelz for building me this cool new before-and-after image display tool! And while I'm thanking people, a big thanks also to the MAHLI team for making such a gorgeous camera and continually coming up with creative ways to use it.
If Curiosity has accumulated dust, Opportunity has recently lost it. Here's another before-and-after comparison of the rover deck taken early this year. In the beginning of the year, Opportunity was so dusty that she was almost camouflaged against the Martian soil. Just two months later, she was much, much cleaner. And the cleaning events have continued; the rover is now nearly as dust-free as she was right after landing.