I saw a clever design for a general aviation GPS the other day. Normally a GPS unit is a rectangular box, the same size and shape as other electronic gear, like nav/com radios, and transponders. Like a car CD player: a rectangular face on a long box that slides into the appropriate spot in the radio stack.
Not everyone has room in their avionics stack for another box, even when they want a GPS. But a lot of airplanes have spots for extra round gauges. The GPS display that I saw was designed to fit and bolt into a round dashboard hole that had previously accommodated an ADF. The ADF is a venerable navigation instrument, still very much in use, but not nearly as accurate, easy to use, or as sexy as a GPS. I'm sure they've been around for years, but it was the first I had seen and I applaud cleverness, even belately.