It's by no means a new idea, but the fact that Google is introducing the new "Sidewiki" feature to the Google Toolbar today means that it may gain widespread adoption. The basic premise, as outlined in a post on the Official Google Blog (with video demo) is that a lot of people visit the same pages on the web looking for information, and giving them a way of adding some helpful knowledge to these pages in the form of annotations means that more people will find what they are looking for. With Sidewiki, if there are notes available on a website you visit, they will appear in a browser sidebar for you to peruse. Or should you want to add useful information of your own, then you can do that too.
Of course, giving everyone free reign to add notes to websites could potentially lead to plenty of 'less desirable' information being added, which is why Google says it has spent a lot of time working on the ranking algorithms involved. Taking into account both user feedback and a number of other signals (naturally, they don't go into specifics), only the most useful or "high-quality" entries will be shown. Sidewiki will also take entries made on one page and display them on other pages that use the same snippet of text. If you want to try out Sidewiki it is currently available with Google Toolbar for Firefox and IE (Chrome and other browsers to come later).