I'm working on dushed[1]. It's a personal knowledge management web app that combines flashcards, bookmarks, notes, web feeds, files and Kindle highlights into one.
Over the years, I've used spaced repetition to learn things and to remember them. But the material for my flashcards came from everywhere: books on Google Drive, blog posts in a bookmark manager, Kindle highlights, Wikipedia, and notes spread across email and notebooks. The cards were in one app and their sources were in five others. This was overwhelming so I'm building dushed. You can access all your knowledge sources(bookmarks, feeds, Kindle highlights, files, flashcards, etc) in one place and also link sources to one another.