Ramps?

You can sometimes find them at greenmarkets or specialty produce markets. They are delicious. Very garlicky, only better and with a "greener" flavor.

They're wild leeks, and while they look a little like scallions, they have more "character," imho. As Panfusine said, their window of opportunity is very brief, although they can be preserved by pickling.

Some restaurants seem to have them longer than they are actually in the markets.

nope, Ramps are a wild variety of onions which are foraged rather than cultivated. They kind of resemble scallions but with a deep garlic flavor & broad green leaves tinged with pink. They're in season for a very short time in spring. Here's the wikipedia entry. Hope it helps
http://en.wikipedia.org/wiki/Allium_tricoccum