View and Set HTTP headers for Safari/Chrome
There are a couple Google Chrome apps that do this. One is called Rest Console.I actually found an app called GraphicalHttpClient in the Mac AppStore that's a lot easier and more enjoyable to use.
The (currently experimental) WebRequest API lets you do view and modify headers: http://code.google.com/chrome/extensions/trunk/webRequest.html
It's pretty easy to view headers using onSendHeaders.
To edit headers, you'll need to block the request. This sample (from the docs linked to above) removes the User-Agent header from all requests:
chrome.experimental.webRequest.onBeforeSendHeaders.addListener( function(details) { delete details.requestHeaders['User-Agent']; return {requestHeaders: details.requestHeaders}; }, {},["blocking"]);