Captivate is an app that does one very simple thing: whenever you connect to a wifi hotspot, it tries to determine if you are behind a captive portal. If you are, it shows a notification to let you quickly open the portal login page. If your session times out, Captivate will re-show the notification when you next turn on your screen.
It automates the process of opening a browser, trying to visit a webpage, getting redirected to the portal's login, then having to re-type the URL, or worse, go back to the app that opened the browser and re-share.
For developers: Captivate sends a broadcast intent whenever the portal state changes. See more on the GitHub page: https://github.com/zach-klippenstein/captivate