I've been using IPvFoo on all my PCs since I wrote it for Chrome 12 years ago. Recently I made it fully Firefox compatible. It's useful if you have IPv6 and want to see which websites are on board, though it's a bit depressing if your ISP only offers IPv4.
I've found it particularly interesting on Lemmy, because it connects to such a wide variety of independent servers:
Nice tool. I've recently started learning about what these networking terms mean. Basic stuff like like what different types of IP addresses there are, how the Internet works, etc. etc. I'll check it out.