1) StatCounter - This is by far my favorite measurement tool. StatCounter provides a suite of measurements far more detailed than what you get from your ISP, like not only the number of unique visitors, but the entry and exit page, where they recently came from, the keywords used used to find the site, the time on site, the number of return visits, and location information detailed enough that you can almost see into the visitors house. You just place a piece of invisible code on your website or blog and away you go. StatCounter only remembers the info for your last 500 visitors, and anything beyond that requires a monthly fee, but the free version is still great.
2) Tynt Tracer - Another tool that requires that you insert some invisible code, Tynt Tracer differs from just about anything else in that it measures how many copies of text or graphics were made from your site or blog. In other words, if someone copies a piece of text from your site and inserts it on another site or blog, Tynt Tracer knows about it, and then can track how many visits came to your site from that copied piece. What's more, you can also program it to insert a Creative Commons license warning when someone copies and pastes your content. The analytics are difficult to decipher sometimes, but it's certainly a cool and useful tool.
3) Google Analytics - There's no code involved with Google Analytics. You just register your site or blog, then leave the rest to Google. You get somewhat the same information as with StatCounter, but you also get some trending information that's very useful.
There are a ton of paid measurement tools out there, but why spend any money until you really need to? These 3 will get you started and take care of most of your measurement needs for quite a while.
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