I started https://omnimemory.com nearly 1 year ago. It's a spaced repetition flashcard system with a minimalist UI designed for both desktop and mobile.

It has a freemium model. It only has 35 signed-up users and nobody is paying. I only did some "marketing" for a couple of weekends (this is a side project), posting about it in some blogs, forums, Hacker News... I quickly got tired of this marketing work and moved on to other projects.