We've written about several annotation tools in the past, but the number of tools and the functionality that they offer seems to have exploded recently. What follows is a review of some tools and resources where you can learn more.

A.nnotate is an online collaborative document annotation and management tool, which offers both paid and free subscriptions for uploading texts to share, mark up, and comment on collaboratively. This tool is excellent for collaborative research and group work and also functions well as a personal document manager. After you sign in and you can upload any number of document types such as PDF, Word, PowerPoint, and various image formats. Notes on documents may be organized by tag, background color, and other display options. Content may be made public or private (via invitation or a public web address). Each set of notes has a hyperlink that allows you to share annotations in emails and on websites (e.g. a course site).

Google throws its hat into the ring with sidewiki. Though not a wiki, it does set up a commenting system on the side of any web page. Comments can be made at the page- or line-level (by simply highlighting the text you wish to annotate). Requirements: a Google account and the Google Toolbar. Runs in Internet Explorer and Mozilla.

Shift Space brings the most powerful and, perhaps, the most subversive set of tools to web-page-annotation. Via a set of GreaseMonkey scripts, the service allows you to place a "layer above any web site". When you push the shift+space keys you can place "shifts" into that page. Shifts include simple annotations (like sticky notes or highlighting), but can also include image swaps and behind-the-scenes code modifications. Of course, in order to see the shifts each visitor must have the software installed and be using the Firefox browser.

For further reading (and to explore other media annotation tools), the Open Video Alliance has an excellent Multimedia Annotations page that highlights many other tools.