The concept of Web Annotation isn't new. Methods of leaving notes on websites for other users of the application to see date back as far as 1999.
The concept is very appealing. Leaving notes that indicate information is outdated or otherwise inaccurate. Leaving notes that expand upon the information that is presented. Leaving comments in the same manner you might jot a note in the margins of a book.
But unfortunately, the more competition there is, the usefulness of each application diminishes, since the user base for each application diminishes.
So until one application is declared a winner, it's hard to know which application is best to use. One might argue what is needed is for a well-established web entity to enter the Web Annotation market and establish themselves as the service to use. And perhaps open their code for others to use so that there can be competing systems that share the same content.
Enter Google SideWiki
Announced this morning on the Google Blog, currently Google SideWiki installs on Firefox and Internet Explorer browsers using Google's Toolbar.
They allow owners of websites to create messages that will always be on the top of the 'SideWiki' for a page. (They can identify what websites you own through which Google Account you are signed into, and what websites are associated with that account.)
And they are offering access to their API (code) so others can write applications that utilize the content left in SideWiki.