Must-have tool for understanding your web site and blog: Google Analytics
I use Google Analytics as a feedback mechanism for what people actually read on my blog and to know which pages on my web site people read (which I equate to which pages people find most useful). As an example, 50% of visitors go directly to my Open Content web page so this gives me a strong incentive to write more free web books and post the PDFs.
As a consultant I interact with many developers and companies and it always surprises me when people don't bother to measure performance, in this case understanding what content people find interesting and/or useful.
Speaking of measuring performance:
Another must have tool if you do deployments: get a free 1-server account at cloudkick.com. (Disclosure: if you end up eventually signing up for a non-free multiple server account then I get a small perk.) If one of your web applications starts to slow down it can save a lot of time being able to tell at a glance what the I/O, CPU, network, etc. activity has been in the last few days.