Google Analytics is a great tool for monitoring the effectiveness of your website as well as individual marketing campaigns.
It's easy to set up and monitoring your sites effectiveness is a breeze. Simply sign up for a free account here - http://www.google.com/analytics and add a small code to each page on your website.
Then simply Login to your analytics account periodically and check the websites performance.
A basic run down can be found below.
 

Fast Implementation
Paste the Google Analytics tracking code into each of your website pages and tracking begins immediately.



Keyword and Campaign Comparison
Track and compare all your ads, email newsletters, affiliate campaigns, referrals, paid links, and keywords on Google and other search engines.
 




Custom Dashboards
No more digging through reports. Put all the information you need on a custom Dashboard that you can email to others.
 




AdWords Integration
Buy keywords on Google AdWords and use Google Analytics to learn which keywords are most profitable to your business.
 




Trend and Date Slider
Compare time periods and select date ranges without losing sight of long term trends.
 




Ecommerce Tracking
Trace transactions to campaigns and keywords, get loyalty and latency metrics, and identify your revenue sources.
 




Funnel Visualization
Find out which pages result in lost conversions and where your would-be customers go.
 




Site Overlay
See traffic and conversion information for every link as you browse your site. (no download required).
 




Email reports
Schedule or send ad-hoc personalized report emails that contain exactly the information you want to share.
 




GeoTargeting
Find out where your visitors come from and identify your most lucrative geographic markets.
Social Bookmarking