I always use http://perdu.com because it makes me smile. It's not even intended for testing purposes.

http://captive.apple.com was my go to just because I'd see it flash in the url bar before redirecting when connecting to airplane wifi. I like the subdomain, it's an apt description of the state your in before passing through the login/payment flow for these kinds of networks.

Chrome's equivalent is http://www.gstatic.com/generate_204 for those curious, and plenty of other browsers have their own.

Neat tools as until I learned of the existence of these ssl-less sites to prompt captive portals I use to try to connect to various sites until it showed up.