I have been using 'Keyword Country' for tracking my traffic and my competitors' traffic. It gives me track of keywords which are pulling maximum traffic.
I like Google Analytics or AwStats, you can Also find many website traffic checker on net - free or paid.
You can use Google Analytics to check the traffic of a site. For day to day traffic google Analytics is more better then otherone.
You jus login in google analytic site. There you may find google analytic code in it. Just paste it in your own site to find the traffic.
You can check traffic of your website in Google analytics. You can even check revenue of your site in analytics.
Get registered in Google Analytic and you will be knowing about the traffic about your site day by day and you can also check it directly on Alexarank site no need to registered there.
you will never be able to truly get an accurate read on a site's traffic unless they release the numbers. Free services like Google Trends and Compete give an interesting and generally fairly accurate idea of the traffic while paid services like comScore, Netratings and Hitwise can get really good data for larger sites but they all will differ from a site's internal figures. Your best bet is to look for visible markers (media kit figures with traffic, adwords impression count if they offer site placements and available web stats) and form an impression of the site's general traffic and go from there