Use case: wait for the first of several urls to complete. E.g.

futures = [ctx.urlfetch(u) for u in urls]
while futures:
    result = yield tasklets.first_available(futures)
    if result.status_code == 200:
        raise tasklets.Return(result.content)
    futures = [f for f in futures
               if not f.done() or f.get_result() is not result]

Hm, the API should make the awkward removal of the selected Future unnecessary 
as well.