Marc Petitjean's Magic Head is something similar to what Micropteris is talking about, but you tie it into the head of your streamer pattern. Pushed back, it folds over the head of the streamer and helps it keep a nice baitfish profile. However, if you pull it forward, it forms a cup that makes the fly chug and wobble. I've used it a few times, seems to work pretty well, especially on patterns worked just under the surface. Only drawback I've found is that it makes your streamer cast like a popper, lots of air resistance.
If it swims and eats, it'll eat a fly.