If you're developing REST based services, Fiddler should not be news to you. But there might be a little trick you can use to do some simple stress testing. If you select one or more sessions in the Web Sessions pane and then hit shift+R you get a little dialog asking for number of times to replay the selected requests. There is no obvious way to find this in the UI as far as I can tell, but holding down shift as you click on Reissue Requests in the context menu does the same thing.