If you've never heard of XRay before, it's this amazing tool to inspect swfs while they run. You can see visual objects, runtime tracing, and even manipulate some of that to see how things look.
XRay Viewer is a simple application that hosts the XRay connector and loads a swf that you can look at through XRay. It's a no-code-change way of using XRay.
If you want the source for XRay Viewer, you should be able to just install it, right click and select "View Source"... but that doesn't seem to work with the latest version of it. Someday I'll fix that but for now, here you go:
It's really nothing special, but it can make working with swfs from designers easier. If you just want to install, there is an AIR install badge on the XRayViewer page that makes it easier than building.