Alexa is the most popular in publicly solicitng user reviews. There are many smaller sites like BizRate and otehrs that do collect user reviews.
IMHO there are more important things than these public user reviews. The Google toolbar collects alot of data, some versions allow the user to specify whether they like it or not. Yahoo's registered users can Save or Block any url in a serp. The big search egnines also use click tracking for many things like to see if the #6 url gets clicked more then the #2 url. I'd be more interested in how the search engines are using this data than in Alexa or the other user review sites.
My question is more about where a person could go to post opinions about a particular site, especially a forum, and it has a chance of being noticed by a fair number of other folks. I'm not asking about the importance of ratings to the well-being of a site.
I highlighted Alexa because I think they draw a fair crowd checking to see what's going on in cyber world -- what's in, what's out; who's up, who's down; who's hot, who's not; etc.
There must be sites where you can go and post something like: "Heh, I stumbled onto this site and it is really cool because ..."