My favorite word of mouth measurement tool is the "most emailed stories" box on any web site. It gives you a true feel for what turns people on. Here’s NPR’s list for 2007.
In many ways, it’s the truest measure of the best stuff on your site — less complicated and biased than any other web metrics.
Why? Because it’s really, truly got to be good if a reader is going to forward it to their friends with their personal endorsement. It’s the true vote of the people.
The NY Times keeps a running list of most email, most blogged, and most searched. You should add this to your site too.