Diigo

Diigo is a research tool and knowledge-sharing community built around a simple idea: bookmarking is not enough if you read to learn. Its browser tools let you highlight text and attach sticky notes directly on web pages and PDFs, and the annotations persist, superimposed on the original page whenever you return, like writing in the margins of a book. Saved pages can be uploaded and archived even if they sit behind logins, and everything is searchable by full text, including your highlights and notes.

Organization runs on tags plus lists, with an outliner for structuring research and group libraries for teams. The vendor claims over 9 million users. The free plan is limited (around 500 lifetime bookmarks and 100 lifetime highlights per aggregator data); Standard is $40 a year, Professional $59 a year with unlimited PDF storage and outliners, and Business is $10 per user a month with collaborative annotation. The interface shows its age and has a learning curve, but for students and researchers who annotate heavily it remains one of the few tools that treats the web as writable. No independent benchmarks are published.

Create a free account at diigo.com and install the browser extension to highlight and annotate.