seeds = ['http://www.foo.com/',
    'http://www.bar.com/',
    'http://www.baz.com/']
threadList = []
URLs = []

for seed in Seed:
    retriever = Retriever(seed)
    retriever.start()
    threadList.append(retriever)

while threading.activeCount()>1:
    sleep(1)
for retriever in threadList:
    URLs.extend(retriever.getLinks())