Captive Portal in Mavericks I connected to a public wifi hotspot with my MacBook AIr (Mavericks) and a login window appeared. I can't recall if I first tried to navigate to a website or not. I am familiar with logging into hotspots, where the login details appear in the browser itself. In this case, however, the login details appeared in a separate window (program?).

When I selected the browser, the global menu indicated "Firefox". When I selected the login window, the menu showed "Finder". Is this a (Apple?) captive portal program? It's the first time I've seen it on any OS platform.
It's something built in the system . Sometimes it's able to detect a captive portal and it displays it in a separate window. 

As @George Garside pointed out, it's a separate program located at /System/Library/CoreServices/Captive Network Assistant.app. 

Also, if you have a iDevice you'll see a similar feature.