NERTS! Online is the free to play card game from the clever people at Zachtronics (Eliza, Opus Magnum, Infinifactory) that was created to help them play together that they released free to the public.
It's not an original game though, as Nerts has been around in some form since the late 1890s. It has also been called Pounce or Racing Demon. Zachtronics mentioned they "learned about it from our artist Kyle, who has played it for years, and started playing it after lunch in the Zachtronics office. Over time we added more to it, including house rules, longer tournaments, and even a theme song".
At release the multiplayer was limited to Steam friends and people you're in Steam Groups with. It worked but now they've nicely expanded it to have public lobbies. Now anyone can download it free (it's properly free too, no micro transactions), create a lobby and make it public for anyone else playing to join.
Seems like a really slick game overall too, great to see more like it and being free means there's no barrier to enjoying it with your friends and others alike.