Apple has been using http://captive.apple.com/ http://captive.apple.com/ for years. Their UI for it has gotten a lot better, too. Previous versions you'd get

Apple has been using http://captive.apple.com/ http://captive.apple.com/ for years. Their UI for it has gotten a lot better, too. Previous versions you'd get a popup, but you could dismiss it and it would disconnect from the network (which sucks if you're /trying/ to connect to a network without Internet). Now it has a few more options and ways to get back to the popup--but I still have issues with certain networks.