I have used a tonne of tools to help me debug web sites on the client side. Even 4 years ago proper HTTP debugging was very difficult and the tools very simple, but a number of tools have come to the rescue. I used to use Firefox Web Developer Toolbar as well as the IE Web Developer Toolbar. But today, I had an issue with a large flash site we deployed for a client in China (http://www.excitechina.com.cn/) running REALLY slow even though its hosted on a heap of servers in 2 separate data centers and has Akamai on top to cache the whole package. Both the Firefox and IE Dev Toolbars fell way short, so I went hunting for something better ... and found it :)
Hello Firebug! This little ripper of a plug-in helped me track the contents being pulled in by the Flash container, and debug a configuration error with Akamai - one happy client (and an even happier client ;)
PS - Been at the Next Digital Shanghai office this week - we're really building a great team with some awesome skills. Took some digi pics and hope to post them next week...