>I like Mnemosyne (homepage) myself - Free, packaged for Ubuntu Linux
It seems to use a 2 decade old modification of a now 4 decades old algorithm which will be worse and waste more of the user's time than using Anki with FSRS or SuperMemo with SM-18.
Based on discussion on HN several days ago, I have started using Hashcards (markdown-based flash cards with FSRS; run from command line, but view cards in the browser). Once set up, it's pretty simple (e.g., no fiddly settings) which helps a lot. Getting new cards added is easy — especially with a Keyboard Maestro macro that I made (global keyboard shortcut pops up a window to enter Q/A and select a deck, i.e. md file, to append to; entering text only on the Q line creates a cloze-deletion card instead). Recommended.
My use case is drilling English --> target-language sentences, as well as law-related knowledge, miscellaneous facts, etc. Still mulling over what to do about other skills-based practices, à la Andy Matuschak's concept of "spaced everything".
If you're drilling English --> target-language sentences with spaced repetition, you might be interested in the free site I made to do just that.^[1] You can find the link in my bio. The source code is also on github.
Sentence practice is really the best way to do things imo. Studying vocabulary in isolation is so limited by comparison. So nice moves there.
^[1]: Actually, my thing does target-language to english drills, not the other way around.
It seems to use a 2 decade old modification of a now 4 decades old algorithm which will be worse and waste more of the user's time than using Anki with FSRS or SuperMemo with SM-18.