OSX and Linux

If you're on OSX or Linux, the best option is the Charles Web Debugging Proxy. It's a great and well-supported app, and while commercial, it is worth every penny. I've looked for good alternatives that focused on web development, and Charles really stood out.

The interface is similar to Fiddler, but it offers two different ways of looking at network traffic:

The style is entirely up to you. I tend to lean towards the structured view because it feels a little more organized, but it is a little more work to find out where a specific URI is.

Like Fiddler, Charles also offers an autoresponder capability. It's called "Map Local...", and you get to it by right-mouse clicking on a specific URI. This allows you to choose a local file to work with.
When I reload the page, I'll now have jQuery v1.2.6 replaced by the local copy of jQuery v1.9 that was on my computer.

Another great feature of Charles is the ability to throttle your network requests to simulate specific bandwidth speeds. I remember the days of 56k modems and their blazing speeds, so using this brings back fond memories (um, right):

Charles can also work on Windows since it offers a complete cross-platform UI.

Which Tool to Use

I use all of these tools all of the time because I test on every major browser. Having this capability really makes troubleshooting easier. Naturally, choosing whether to use a browser-based sniffer or a hard-core app-based proxy depends entirely on your debugging needs.

If you just have to inspect some traffic and check results, a browser-based sniffer will most likely suit you perfectly.

On the other hand, if you need granular control of how URIs respond or want the flexibility to create custom test scripts, then a tool like Fiddler or Charles is where you need to go. The great thing is that we have solid choices to help us do this, especially as the complexity of our projects increase.