[MauranKilom]

Most recently, I was using bash.org as my go-to non-HTTPS site for captive portal purposes (back when HTTPS Everywhere was a useful extensions). But the built-into-Firefox HTTPS only treatment handles captive portals gracefully already, so I didn't actually visit in a while.

Goodbye!

[meatmanek]

I use http://alwayshttp.com for this.

[Titan2189]

https://neverssl.com/

[p1mrx]

http://http.rip/ is better, if you want https to actually fail.