Polar is described as 'powerful desktop app for managing all your web content, books, and notes. Polar keeps all your content in one place, supports tagging, annotation, highlighting and keeps track of your reading progress' and is a popular app in the News & Books category. There are more than 50 alternatives to Polar for a variety of platforms, including Mac, Windows, the Web, iPhone and iPad. The best alternative is calibre, which is both free and Open Source. Other great apps like Polar are Google Keep (Free), Pocket (Freemium), Evernote (Freemium) and Zotero (Freemium, Open Source).