However, usage and referrer logs will not give you information about search engines in a very easy to understand format. There are several such tools on the market. I'm a moron when it comes to cgi, programming, parsing, all that junk. WebPosition Gold provides you with all that you will need to increase your ranks, including tools that analyzes your pages for appropriate keyword density for specific search engines. WPG covers all the major SE's and directories.
Price varies according to traffic.
Web Trends provides you with TONS of data. A little too much information for my uses, but if you are going to implement a heavy-duty marketing campaign and you need all the nitty gritty statistics, you'll probably get the most bang for your buck with Web Trends. It's free if you don't mind a little button on your pages. Otherwise, like WebTrends, price varies according to traffic. Like WebTrends, provides you with oogles of information about your visitors.