Google +1 has been quietly rolled out over the last few months to Google users - allowing them to "+1" (aka, "Like") pages found in Google search results. Today, +1 has been expanded into a number of other Google services, including the Android Web Market:
Pretty neat, right? ROM Manager likes are already piling up. It's very likely we'll see +1 expanded onto the Android Market app in the future, as Google tries to make +1 a part of everyday life for Google users.
Additionally, YouTube, Blogger, and Product Search all have implemented +1. Google has announced a number of partner sites that will have embedded "+1" buttons for content and products (eg, Best Buy, Reuters), similar to the way other social sharing features are incorporated (like Tweet buttons).
+1 itself is sort of a combination of Facebook's "Like" button and Twitter's "retweet" functionality - you can see when your friends +1 something like a webpage, but in certain contexts (like the Market) you can see how many people total have liked something (a bit like Facebook's "x people like this," but public and anonymous).
Get all the info on +1 over at the Google Blog.