These tools can be used for search engine optimization (SEO), or to obtain statistics about the types of visitors your site receives.
Google Analytics - website statistics tool (daily hits etc..), now integrated with Google Adwords - uses JavaScript snippet on each of your pages to track visitors and trends
Google Page Rank Checker - check your Google Page Rank, which is used to position your site in Google
Alexa Traffic Ranking - check the popularity of your site (click Traffic Ranking and enter site name)