Fiddleheads are the newly emerging, still tightly coiled, fronds of the Ostrich Fern, named for the fact that they resemble the top of a violin or fiddle. They're only available in the spring & have been showing up for the past month in farmers & small upscale organic markets, as well as some supermarkets (where I purchased mine yesterday).
I love them, especially since I love variety in my vegetables, & to me the fiddlehead fern epitomizes spring. Only the Ostrich fern variety is considered edible, & the taste is very mild with a slight earthiness.
Here's a link to an online company that sells them. When they run out of fresh ones, they sell them frozen as well.