Diigo
FreemiumDiigo is a comprehensive online tool designed for researchers, students, and lifelong learners to collect, organize, annotate, and share information found on the web. It acts as a powerful bookmarking service combined with robust annotation capabilities for web pages, PDFs, and documents.
                  
                    
                  
                  Key Features
                
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                      Comprehensive annotation tools for web pages and PDFs.
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                      Reliable webpage archiving prevents link decay.
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                      Powerful organization system using tags and lists.