Readia's answer:
Readia focuss on "Retain what you read." A user can take an exam from his Kindle highlights and set a schedule for reviewing.
Readwise - Effortlessly rediscover and organize your Kindle highlights
Knotes - An efficient, beautiful Kindle highlights & notes manager
Booky - Online bookmark manager. Store your favorite bookmarks online. Available everywhere. Private. Customizable. Fast.
Pocket to Kindle (P2K) - Send articles saved in Pocket to your Kindle
Clippings.io - Organize the notes you make on your Kindle
Klib - Kindle & iBooks Highlights Manager