There's also one from Fitbit's founder, Texts app - this space is getting crowded!

https://texts.com/

There seems to be little information about the company/people who runs Texts on the website which is strange - where did you find the info of who runs it?

My bad - Beeper is the one that's founded by Pebble founder (not Fitbit). Kishan Bagaria is the one behind Texts.

https://twitter.com/KishanBagaria

I can't find any link between Texts and Fitbit, but I know that Eric Migicovsky founded Pebble (acquired by Fitbit) before founding Beeper.

Ah my bad - I must have mixed them up.

You can also self host it for free according to their website.