Sea Beans! I've found only at farmer's markets although I know there are other places that have occasionally carried them. I have so much more to say about these green, crunchy haricot verts of the ocean but I've been swamped this week. All I can say is that if you find these fantastic sea beans or sometimes called drift seeds, pick yourself up a bag! See this great link about sea beans for the information I wanted to give to you but don't have time to fully share. The Ballard Farmer's Market is where I saw them last.
Quick Tip on Cooking Them: Blanch in boiling water, stop the cooking in an ice-bath, toss lightly in olive oil and either eat them as is or them make a great salad or Asian soup addition.