Scrible is an excellent tool for annotating web sites. Non-subscribers can see your annotations, but they do need to sign up to add their own comments. It’s free to sign up and those with an education email address receive a larger storage allowance.
Scrible can also be used to store web site links and is able to generate an academic reference in most common formats (Harvard, APA, MLA, ACS) as well as being able to export to EasyBib. these libraries of links can also be shared. Links and annotations can also be posted to Twitter or Facebook.
These feature make it useful either to share your assessment of a web page, provide feedback on student’s web sites, share links with students or for students to make their own comments, individually or as part of a group.
There are apparently plans to add PDF annotation at some point, but this is not yet available. There is an iPad version too.
Click the image (which shows editing in progress) to visit an annotated page.