do you mean an adapter that allows 37mm lenses to be placed onto your camera?
you can get a lens filter, i am using a skylight filter which makes outdoor pictures not so overbearing with respect to the sun. it's also my current digital camera's lens protector ;-)
i also have a linear polarizer, which allows me to take good pictures where there would be glare. that is, if i want to take a picture of something underwater (clear and clean water like in the caribbean), i can without the reflections cause problems.
anyway...
you can also get some telephoto or wide angle lens to enhance the optics of the camera.
or you can get special effects lenses, but you can do that digitally anyway...
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i'm very green with respect to photography... ;-) but that's the gist of anything i know...
basically, the most important thing the filter i got is to protect the lens of my camera.