Last year, Google launched a feature called SearchWiki that allows users to customize search results. If you are logged in, you can remove search results, promote them at the top of the search results page and enter comments. While the feature is useful to personalize the results for frequent queries, the "wiki" component was only an afterthought.
Check the SearchWiki page for "google" and you'll realize that the 27511 notes recorded by Google aren't very useful. Comments aren't helpful, even though Google tries to rank them by usefulness.
A similar feature is now available in Google Toolbar. Google Sidewiki lets you enter comments about any web page and shows some of the best comments in a sidebar. The feature is integrated with Google Profiles, so you can find more information about the author and read other Sidewiki comments.
Google notifies you if there are comments about the current page, so you need to send your browsing history to use the feature.
Sorting the comments by date wouldn't be a great idea, because spam and silly comments like "lol" or "cool site" would be prevalent. That's why, Google developed a ranking algorithm that takes into account many signals: user votes, author's authority, text analysis. Danny Sullivan says that "Google has a language sophistication detector now, and one that works in the 14 different languages that Sidewiki supports".
Learning some information about a site, finding if a certain company is reputable or reading a comment that corrects some errors from an article - all are use cases for Sidewiki, but it remains to be seen if Google manages to rank comments properly.
As with Knol, Google encourages experts to post comments in Sidewiki: "What if everyone, from a local expert to a renowned doctor, had an easy way of sharing their insights with you about any page on the web?" Unfortunately, experts don't have an incentive to post comments and isn't always easy to distinguish experts from opinionated users.
Larry Page once said that Google wasn't supposed to be a search engine. "We built a ranking system to deal with annotations. We wanted to annotate the web--build a system so that after you'd viewed a page you could click and see what smart comments other people had about it. But how do you decide who gets to annotate Yahoo? We needed to figure out how to choose which annotations people should look at, which meant that we needed to figure out which other sites contained comments we should classify as authoritative. Hence PageRank."