Google Analytics is a great free service, which allows you to track the number of visitors and other detailed statistics for you website. I install it on all new websites. If you have a Google account, I will share your site stats account with you so you can view them online anytime you want. Whether or not you have a Google account, I can set Analytics to send you a monthly, quarterly or yearly report of your website statistics.
This is a useful WordPress Plug-In that puts your unique Google Analytics tracking code on each page of your WordPress site:
Google Analytics – Enables Google Analytics on all pages.
Version 1.0.5 | By Kevin Sylvestre | http://wordpress.org/extend/plugins/googleanalytics/