Large Open-Source Based Web Sites?

nellardo asks: "I know that, these days, most web sites are running on Apache and many are running on Linux or some flavor of BSD as well. But what are the biggest sites that go beyond that? We've talked about open-source application servers and the like here before, but where are these being used? Why do I ask? Well, duh, I'm trying to convince management :-) Seriously, CMP uses Mason and Slashdot uses Slashcode, but what are some of the other large ones?"