Google Analytics
Google Analytics is an invaluable tool for analysing the traffic coming into your site, but most people barely touch the surface..
Its dead easy to set up, really quick to learn the basics and has some incredible powerful statistics that it can generate by linking it in to ecommerce sites. It can tell you home much revenue ppc campaigns are generating you, how much traffic you get from search engine results pages (and what keywords they are using).. even where in the world the traffic is coming from and how much each country is making you.
If you have an ecommerce site, you would do well to start using it.
