Google Analytics is a fantastic webstats service. I would pay for it, but I don't have to; its free. You simply add some code to your website pages and the data is gathered and presented in easily understandable graphs and tables. Every website owner has to understand what their visitors are doing. This service will give you that insight. - Read more

CrazyEgg.com is a good tool for tracking what your website visitors are clicking on, on each of your webpages. This is critical information that allows you to continually improve your sites usability and content. It costs between $9 and $99 per month. It is well worth signing even if you just run it for a month. - Read more

SpyFu allows you to download the adwords and keywords for your competitors and see how your own efforts look in comparison.  Very handy!

SpyFu has also now introduced SpyFu Kombat, allowing you to compare two or more competitors against your own website, and compare keyword overlap and other key metrics.  - Read more