Google has another tool within Google Webmaster tools. Go to your site within Webmaster Tools, click on Labs, and then Site Performance. This will show you what Google believes to be your sites overall performance and average load time. It will also list suggestions on what pages are slowing you down. The tool is relatively new and as such isn't great.
Microsoft provides a tool within the SEO Toolkit that is now included in IIS7. When you do a Website Analysis of your site it will tell you how long it took to load each page.
Zoompf & MSFast/MySpace’s Performance Tracker are two other notable free web performance analysis & optimization tools
What I've gotten in the habit of doing is loading up firebug, which measures every file loading to the ms, and then jumping on a proxy like Tor. Then I can see in real time which files are taking too long to load in say Germany or Australia. Usually at throttled speeds not much faster than dial up.
I'm sure the other serveces mentioned are great but this is a real world way to do it.
Octagate's Site Timer provides a graphical view of the load time if you want a web site, or you can get similar statistics with Chrome and Opera's developer tools.
gtmetrix.com is similar to webpagetest.org, but another option as well.
There's a very good metric that never fails for when we tweak (or sometimes un-tweak) our highly successful website. If the pages are even 0.5 of a second slower, sales drop by up to 30%. Sounds crazy, but it's true. When I remember to enable GZip compression and minify our JS/CSS and switched to sprites our sales went up.