A very good Tool.

Web-Stats is FREE for the first 30 days then 5 dollars per month.It provides precise, detailed information about each site visitor, where the person is from, how long the person stays on each page, entry pages, exit pages, key words used to reach your website, referrals, etc. so you know exactly what advertising techniques are working for you and what you should improve on.
You can get many free counters from different company but if you want them invisible you have to upgrade and pay over $5 and also sometimes you can start your counter where ever you want so it's not very reliable if you need to proove how much traffic you are getting.