Following on from http://stackoverflow.com/questions/53738/how-do-you-test-the-performance-of-a-website :
What tools/sites/services do you use to find out the performance of your website from multiple locations around the world?
There are always complaints that sites are slow under certain circumstances (especially from physically-distant places), and it would be nice to be able to tell where it is particularly slow from.
You could try this:
http://www.dotcom-monitor.com/task_instant_test.aspx
It's a free service that tests from a limited list of locations worldwide, but it's a starting point at least.
Pingdom are good, and quite cheap. They'll tell you if your site has suddenly started getting worse from one part of the world.
Gomez are excellent (they can test loading of all the images/css/js etc as well as the HTML itself, using the same pipelining rules as browsers use), and they can test not just from datacenter locations but end-user broadband connections - but they're very expensive.
It's also worth using a tool like YSlow to do basic analysis of your app and point out the things that are likely to cause problems over slow network connections.
You can get users to test your site for very cheap prices by offering tasks on Amazon's Mechanical Turk.
I used this site to check http://www.check-info.com , there have many tools for webmasters