Readwise - Effortlessly rediscover and organize your Kindle highlights
Raindrop.io - All your articles, photos, video & content from web & apps in one place.
Knotes - An efficient, beautiful Kindle highlights & notes manager
Pocket - When you find something you want to view later, put it in Pocket.
Klib - Kindle & iBooks Highlights Manager
Tagpacker - A free tool to quickly collect, organize, and share your favorite links.