Fleck and Mojiti are two online annotation tools that I've been exploring. They help web surfers to refer others to precise points in a web page or video respectively, as well as to add descriptive notes.

Fleck can be deployed in several ways. You can start annotating other websites by putting the desired URL into Fleck's input field. There are other ways of integrating Fleck with your browser, from bookmarklets to browser extensions (the latter method requires registration, but the service is free).

You can see an example of a Fleck-ed page here. (Wait a while for the annotations to load.)

As you can see, once you annotate a page, Fleck provides a unique URL so that other users can see the annotations. The annotations exist on a separate layer from the website. It does not cache the website. That is, if the website changes, your annotation might not point to the right content.

Nevertheless, it's a nifty tool.

If you haven't clicked the Fleck example above, please do so. I've annotated Mojiti's front page to tell you more about what Mojiti does. In case you don't have time: it allows you to annotate online videos.

UPDATE 22 Feb | Just came across this via LifeHack. zpeech allows you to run a discussion/comments thread about any website on the website itself.

UPDATE 11 Mar | TechCrunch points to four other annotation services besides Fleck.