Most hotel wifi networks require opening a browser to connect. Once you open it, the first page you go to is redirected. Because a lot of sites default to HTTPS, that redirect is detected as a MITM by the browser, and prevents you from moving on. http://neverssl.com/ is an easy way to get to the portal without an issue.
As noted on NeverSSL:
> [...] it [...] means that if you're relying on poorly-behaved wifi networks, it can be hard to get online. Secure browsers and websites using https make it impossible for those wifi networks to send you to a login or payment page. Basically, those networks can't tap into your connection just like attackers can't. Modern browsers are so good that they can remember when a website supports encryption and even if you type in the website name, they'll use https.
All the other replies to your comment are "this is a permanent solution to the problem"; but I think to truly answer the spirit of your question, the correct answer is "yahoo.com": I mean... can you imagine Yahoo! ever getting around to adding SSL to their website? ;P
I use ipchicken.com, works over http for public wifi signins, and has added benefit of showing me whether I’m connected to wifi/4g/vpn via IP’s reverse DNS.
Redirects are fine as long as they aren't HSTS preloaded - you can open it in an incognito window if your browser has cached the redirect.
I usually use gstatic.com, which does redirect, but also gstatic.com/generate_204 is Chrome's own capive portal test page and does not redirect. There's also msftncsi.com, which (/ncsi.txt) is Microsoft's test page.