Nothing like as small as the ones Moresound posted about but I use tons of these for all sorts of things:
Admittedly, it's mainly live but you can hide them in the hairlines of actors in plays and musicals, clip them to ties or lapels for corporate stuff (or TV)--and they also work well taped to the back of the neck on a double bass or cello, dangling just below the end. Their LF response is remarkably good for the size.
...so I'm curious about live theatre applications for the ones Moresound found...you could REALLY hide those ones!