A decent place to get a traffic estimate of a site is just google trafficestimate It's not 100% accurate, but it's good for just knowing.
- alexa.com - compete.com ...and many others But these are mostly useful to compare a site with another similar site. The number of visits/visitors/pageviews is very inaccurate. Do not compare the Alexa rank of a health forum for example with the Alexa rank of a webmaster forum. All the third-party traffic estimators are toolbar dependent, so if a site is visited by 1,000,000 users that aren't using the Alexa toolbar, then the traffic is not known by Alexa. The same is available for all the other estimators.