I buried this information between numerous pretty pictures in a yesterday's post, so let me now tell you a little bit more.
A couple of days ago, a new feature was introduced on all published papers on PLoS ONE. Along with commenting on and annotating each paper, you can now also rate it.
The rating system is familiar to all of you from other sites, I'm sure - it is a simple five-star system. You can rate papers on three criteria: Insight, Reliability and Style and let the software average your three ratings to produce a single number, as well as average your ratings with other readers' ratings to produce a single number for each of the three criteria.
Read the Rating Guidelines first, then dig into the site and rate every paper that you read, while leaving a very brief explanation of why you rated the way you did - no need to get wordy, the Discussions comments and Annotations are designed for that.
For a more technical account, read what Richard Cave wrote about the new application, and then check Pedro's post for a quick pictorial tutorial. If you have the Javascript turned off, you will need to turn it on in order to be able to rate the papers.
As Chris Surridge says (after explaining what this all really means): "Never read a paper on PLoS ONE without leaving a rating!" I strongly second Chris on this: go look around PLoS ONE, read the papers (or find those you have already read in the past) and rate them. Now.