Google is testing a "helpful?" button in the Google reviews listings. When you go to a local listing and click to browse the reviews, some are now seeing under each individual review and thumbs up with a "Helpful?" button that is grayed out. If you click on it, it means you think the review is helpful.
Here is a screen shot from Conrad O'Connell:
I personally do not see this and cannot replicate it but Google does let me sort by most helpful reviews and I do wonder if they've been using this sporadically for some time.
Forum discussion at Twitter.