Got a new toy for my guitar, the O-Port. This is, as the image shows, a piece of flexible plastic you stick into the sound hole of an acoustic guitar (the flared bell part down.) Loosen the strings, pinch the plastic, stick it in, line it up, pop it into place. My classical's sound hole is a hair small, so the fit isn't exact, but it works okay.
Once it is installed, unless you know to look for it, you probably won't notice it.
What does it do? It amplifies the sound a bit. Not that much, and the descriptions get subjective, but to me, it sounds fatter, a bit more punch, and more sustain, especially on the harmonics.
An engineer at Bad Bear Studios did a test on it, and it's not just subjective, you can see the graph's differences.
It won't turn a crappy guitar into a great one, but it certainly enhances mine.
The price on these is all over the map, depending on whether you the small or larger version, black or ivory color. Some sites have them for thirty bucks. I got mine from Amazon.com (GuitarGoods partner) for just under fifteen bucks–and that included the shipping.
For that kind of money if you don't like it, toss it, but if it does for your axe what it did for mine? You'll keep it.