Booxia provides spaced repetition for your Web & Kindle snippets. Review later online and by daily email.
Readwise - Effortlessly rediscover and organize your Kindle highlights
Klib - Kindle & iBooks Highlights Manager
Clippings.io - Organize the notes you make on your Kindle
Unhoard - Rediscover things from your Pocket every week
Nimbooks - Host all your digital reads in one place
Hazel - Lighweight update server for Electron apps