When Google launched SearchWiki in November 2008, it came with a little comments bubble, where users can leave comments about a specific site in the search results. Google seems to be changing that comments bubble or icon into simple text that reads "Review This Site."
Here is a picture of the old version, notice the little comments icon on the right of the "Similar Pages" link:
The new version has a plain and simple "Review this Site" link:
Clicking the link or icon (depending on what you see) will open a text box:
Then after submitting the comment, you will be able to see it under the result:
Forum discussion at WebmasterWorld.