JumpKnowledge is a Web annotation too that lets you comment on and share any web page.  You can add comments next to any text on a page and even create multiple pages of annotations.  You can then share these pages with a link, through email, on your blog, or just save them for yourself.  The only beef I have with JKN (JumpKnowledge) is that you can’t comment on images.  But hey, it’s only in beta 2.  Here’s my screencast tour of JKN:

Flash Demo3m29s

(source:  CenterNetworks)