Locate the sender of an email and their geographical location, including messages sent through anonymous email services like Gmail, Yahoo, and Hotmail.
If you want to redirect to a different version of a file based on the time, this code is for you! Please read: Serve Alternate Content based on Time
If you examine the preferences dialog of any modern Web browser (like Internet Explorer, Safari or Mozilla), you'll probably notice a 'cache' setting. This lets you set aside a section of your computer's hard disk to store representations that you've seen, just for you. The browser cache works according to fairly simple rules. It will check to make sure that the representations are fresh, usually once a session (that is, the once in the current invocation of the browser).
Learning about charset's and file types maybe pretty boring, but using .htaccess it can be fun! Here's a quicklist htaccess cheatsheet for adding the correct Charset to a web document.