CritLink is the Web's first universally-accessible public annotation system. It allows anyone (using any browser on any platform) to attach inline annotations to any public website. No client software needs to be installed, since the service is a mediator. To start making annotations, simply go to http://crit.org/ and browse the Web from there.
Annotations are first-class Web documents. As a result, they may be stored anywhere, password-protected or searched just like any other Web pages, and can themselves be annotated. You can download the code if you want to set up your own annotation server (for example, this would allow you to keep a private set of annotations in a password-protected area, or to provide annotation services behind a company firewall).
This project has the following developers:
New HTML Parser: The long-awaited libxml2 based HTML parser code is live. It needs further work but already handles most markup better than the original parser.
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If you're a C programmer with some spare time, take a look at the mod_virgule project page and help us with one of the tasks on the ToDo list!