You might be thinking of a clip-on tuner. I think raecarter is talking about Cherub brand clip on pickup for recording or amplifying ukes. I've seen them where they clip on the head stock and ones made for violins that clip on the sound hole. The ones for the headstock gave a really metallic distorted sound, and I haven't heard the other kind on a ukulele.
Check out K&K Big Shot pickups. They stick a small element on the soundboard and a jack on the side that can be removed if you want to get rid of it down the line. they sound great (I have one on my soprano flea and use it on stage all the time).