Peer review could be hard for two reasons: 1) no contribution, and 2) too many contribution. You could have modules no one cares to write review for. Or you could have modules that have way too many reviews.

Rating system (unmoderated probably) is a good feasible compromised solution. Just as people rate on movie online. I don't know how philosophically agreeable or disagreeable it is to other people.

SourceForge and some other open source portals have download and activity statistics for every project, which give you some indirect hint about the product. It's not a technically difficult to implement, just a matter of whether people like it or not.

Another mindless indicator could be querying how many times a module being used by all other modules. Of course, whatever indicators you used, they should be compared preferably within comparable modules (it won't be fair to compare the popularity of Carp to, say, PARI).