Research tools:
1. Alexa, of course. This will give you Alexa’s score of the site’s popularity. Anything >100,000 is useless since the sample of traffic is too small to discern a proper placement.
2. Compete – also scores the site. Different algorithm, same idea.
3. Quantcast – another score, different algorithm.
Alexa and Domaintools.com will give you a geographic breakdown of the traffic sources to the domain, useful when looking for int’l bizdev.
Compete offers some keywords users use to find the site on search engines.
Quantcast tries to estimate #s on demographics of users (male, female, income, etc)
If you build a matrix of sites you are interested in and then figure out where you can target and be successful, you can turn the matrix into a sales dartboard.

Alexa, quancast and the rest are using approximations to give their answer – it is not accurate, but can give you a good ballpark idea. To make matters worse, some sites can put requests to not be indexed by these services, so there will be no data reported.
I think a better measure is to generate a list of keywords, and then use a tool like seo elite to find out where they rank in yahoo google and msn…and then do the same for your site. It will give you a very good idea of the natural traffic they are getting.
And if you want more traffic to your site, I found a handy free reference at http://websitetraffic.jediwealth.com

Ya every one is right here, you should try alexa for check web ranking of others. http://www.kumaun.in

try to use sitemeter, well I guess other people web traffic is other people privacy maybe some of them have personal data about it and dont want to share their traffic details…