One of the most useful Firefox plug-ins I use at Firefox browser is SearchStatus. It is the great tool for SEO/SEM (I also like very much SEO Quake in Google Results), but I use it most for the evaluation of the Web sites I found interesting, but I do not know much about them yet.
In new version, this tool includes the visual manifestation of Compete site rankings, beside standard Google PR and Alexa Rating. Compete rank is interesting in many ways, but the most interesting for me is the estimation of the number of unique visitors per day (it can be seen on SearchStatus with the right click on the Compete bar).
As I tested for the several sites I know the exact number of visitors, the results are mostly very similar to the real numbers (of course, that is only the estimation, or indicative value). In addition, for me it is very interesting to determine if the site is not older then one or two years, because Compete ratings are given only for older Web sites.
To conclude, I recommend SearchStatus even more after they implemented Compete rankings.