What's an easy way to see what AJAX calls are being made?

Firebug should show it in the Net panel.

EDIT: Also, if Firebug slows down your Firefox in a way that makes it unusable, like it does for me sometimes, WireShark (formerly Ethereal) will analyze all network traffic on your system, including HTTP and AJAX requests.

Fiddler 2 is a great tool for watching HTTP traffic.

* 2014 Update *

Since my original post, both Internet Explorer and Chrome have added built-in developer tools that are quite useful. While I still support and use Fiddler for non-web related http monitoring or when I need to really dig in deep and modify requests in transit, these days I find myself pretty much using IE or Chrome's built in tools as they are sufficient for 99% of my needs. They both have a networking tab, where you can monitor http requests, plus other nifty features for debugging webpages.

• Internet Explorer F12 Tools Introduction
• Documentation for Chrome's F12 Tools
• FireBug, which is mentioned above is also a good tool if you're using FireFox.

I have used nearly all the big tools out there (fiddler, firebug, httpfox, live http headers). The best one in my opinion is HTTPFox if you are using Firefox because it has easy filtering capabilities, allows you to view your JSON the request/response as raw or formatted and the over all interface is very clean.

Also, I can't do without WFetch for testing web services and AJAX stuff. Usually I'll take the request from HTTPFox and paste it into WFetch. This way I don't have to go through the browser each time. I just hit F5 in WFetch to replay the request and I directly modify the request.

Side note: In WFetch, be careful about content-length if you have a POST payload. If you modify the payload the content-length needs to be changed as appropriate. For this reason I save my requests in a file and point WFetch to the file. In the file itself it is easy to know the length of the line and change content-length to that.

_{(source: koreus.com)}

alt text http://www.port80software.com/assets/images/ld_wfetch3.gif