I'm EXTREMELY happy with my new armrest from Armrest Kingdom. At $109.99, it is one of the better-priced options, and it matches the Smart's upholstery well. (Not perfect... the smart has contrasting stitching.
I first saw these at the dealership in Knoxville when I stopped to buy the CD holder while on vacation there. I think they wanted $149 or even $179, so I didn't go for it then. Problem was, I found myself wanting an armrest every time I'd get stuck in traffic, so I began to do some research on the internet and discovered what appears to be a direct source (info below).
Installation took maybe 10 minutes, and most of that was getting up the courage to tap screw holes into my pristine smart Cabriolet. I gritted my teeth and went for it, and I couldn't be happier. It's a little awkward to navigate it over the parking brake handle and between the seats and seatbelt apparatus, but once it's in the spot you want it, there are four self-tapping screws with nice little cosmetic covers to keep everything looking very professional. A phillips screwdriver is the only tool needed.
I'd give it 4 out of 5 stars on my own rating scale... stitching to match the Smart's, and a bit heavier plastic for the base would be necessary for it to max out in my (admittedly picky) judgement.