There is also SuperMemo 18 (https://super-memo.com/supermemo18.html) which is from the same people as supermemo.com except it's a desktop app with way more features but also a much higher learning curve.

If you use SM and you use anki both for a significant amount of time it's not hard to see that SM leaves you with way less reps. Anki is based on the very first version of the SuperMemo algorithm (from ~1990 or so which was the first SRS algorithm ever) known as SM2. Current version of SM uses SM18.

Too bad SM is Windows-only and desktop-only. The beauty of Anki is that you can review anywhere.

On linux you can use SM via vmware/wine and on os x it's quite usable with parallels. If you're using SRS just for language learning then being able to use on mobile can be helpful but I think for those cases supermemo.com can be a decent alternative. More generally, I think the platform specificness of SuperMemo is more than made up for with its incremental reading functionality (alongside better scheduling algorithm)

> On linux you can use SM via vmware/wine

To add to this, there's a set of Winetricks recipes for running SM on Linux.[0] (Disclaimer: I haven't tried them, though I've been planning to, for a while.)

[0] https://github.com/alessivs/supermemo-wine

You might also try my website (and soon to be app) Seedlang: http://www.seedlang.com. It is a site for learning German, and later this year Spanish, French, and English. It uses video flashcards to create different learning experiences, and ties them all together with a Anki-like review deck where any word or sentence encountered on the site can be saved.

Isn't Anki based on Supermemo?

yep, it's based off the first supermemo algorithm, SM2