Bao la Kiswahili: https://www.kibao.org/kanuni.php?lng=en
Not my cup of tea, but I am under the impression that it is considered one of the deepest mancala games.
You can play correspondence on the same site.
Arimaa: http://arimaa.com/arimaa/
An elegant game that looks similar to chess on the surface, but plays more like go in the sense of simpler piece rules and strategy-tactics balance. (more whole board thinking and recognizing shapes and less brute force reading).
The same site is the best place to play it online currently.
Thue-Morse Go: Thue-Morse (Fair Sharing) Sequence: A possible alternative to komi?
As long as we’re listing go variants, I must submit this love it or hate it gem. 
The best place to play online is a site called OGS some of you may have heard of.