Get web server response time

My friend asked me what is the easy test a web server's response time for a specific html file. I browsed on internet a little and found a firefox plugin called "Yslow".

The YSlow analyzes web pages and tells you why they're slow based on the rules for high performance web sites.It will also display the response time in the status bar on the right bottom of the window.
The response time shown by YSlow in the status bar is the number of milliseconds between the onbeforeunload and onload events, in other words, the time between when the previous page unloaded and the requested page was done loading. If you download components after the onload event, those aren't included. Sometimes the response time isn't shown. This typically happens if there was no previous page, such as the first page viewed after the browser starts or after a browser tab was just opened.