While looking for a decent, working, iPod-capable stereo to put in our car for some upcoming travel, I found a really neat feature on the Circuit City website. You tell it about your car, and as you browse the site, there is an indicator beside every car audio product that indicates if it fits in your car.
Very helpful, and very logical. I have no idea if it was designed by 37signals, but it sure looks like it to me. In the same way that one recognizes the work of a good artist, or at least the influence of their works, so we see more and more examples of this kind of smart design around the web.
Photo by Richard Smallbone.