WordPress.com Stats for your WordPress Blog
. . . even if it’s hosted elsewhere.
You can find a blog post on this here, and the download for the plugin here.
I’ve installed this on my non-commercial blogs (that obviously excludes this one), and thus far it looks like it will provide helpful information without slowing the blog down. You do need a WordPress.com API key, which means you will need an account with them, but that is easy to get.