Google Analytics is the best available tool for webmasters and bloggers today. It is absolutely free, you just need a Google account to use it. You can assign your blog to Analytics and check the details of your visitors and the amount of traffic. You can earn through AdSense, but how to increase your AdSense earnings? Yes, it is Analytics tool that helps you to increase the traffic and thereby earnings to your website or blog. You can know the sources of visitors, entry keywords, exit rate and much more. This analysis allows you to make your site/blog healthier. Using effectively in my sense is just keeping the existing visitors as it is and increasing new audience, you shouldn't loose visitors. There are several other tools similar to Google Analytics like statcounter, webtrends, omniture etc.