Readwise - Effortlessly rediscover and organize your Kindle highlights
Bookstash - Bookstash is a free library of curated summaries from top nonfiction books.
Clippings.io - Organize the notes you make on your Kindle
wits.io - Weekly book summaries recommended by founders and makers
Klib - Kindle & iBooks Highlights Manager
Bookcelerator - A curated directory of books that will make you smarter