Create simple user tracking in print ads using the QUERY_STRING
You're planning on advertising in a number of magazines, web sites, and newspapers, and you realize that you'd like to gauge how much traffic is coming in from each advertisement — it'll help you better spend your money in the future. Unfortunately, you have no time to implement something complicated, so you're looking for a low-tech solution, as well as something that will make it easy to analyze the results.
Apache happily logs all environment variables passed to the pages it serves to the outside world. Thus, the hack is simple: pass an advertisement-based QUERY_STRING to the main page of your site. The QUERY_STRING will be ignored if you don't specifically act on it (with SSI, PHP, CGI, etc.), but Apache will still log the information to it's access_log.
For example, say you're advertising in the New York Times. Instead of saying "Come visit us at http://www.GatesMcFaddenCo.com," change the advertisement to specifically match the readership:
In the above example, anytime someone types in that address, Apache will serve the main page normally but silently log that a QUERY_STRING of "nyt" has been passed. Using a log analyzer (like analog, or the simple one below), you can then find out how many people visited your site from seeing your ad in the New York Times.
Since you're using QUERY_STRINGs, you can also act upon them with Server ...