A chrome extension to redirect URLs based on a set of rules. Essentially it rewrites the URLs as the requests are being made, so you can test websites. The original use case was to test local changes to a common javascript library used accross a set of public websites.
A more comprehensive tool to do this is the Charles proxy. For serious work I would recommend that tool over this one, all though the approach is obviously completely different.
If you want to use this simple tool, check out the Chrome Plugin Page.