I use Google Analytics to track visitor numbers to my site as well as a custom written referrers package some of the early users of WordPress.com may remember. That only records 7 days of data because of the data size so when I wanted to know how many visitors come from Google to my blog I went looking at Analytics.
To do the same on your site, open Google Analytics and select your site. Click on “Traffic Sources”, then look at the list of top sources. Chances are Google will be near the top of that list. Click on that link and you’ll see a graph like the one above. Done!