[cimnine]

I still prefer ifconfig.co, which is open-source (BSD-3), supports IPv6 as well and has a nice JSON response as well.

[markild]

Others have posted alternatives, but I really like the fact that you can self host it:

https://github.com/leafcloudhq/echoip

[actionfromafar]

This is the upstream repo:

https://github.com/mpolden/echoip

[mobilio]

or ifconfig.me

[petecooper]

or ifconfig.io