Fiddler is a very useful tool for troubleshooting all kinds of HTTP(s) communications. It also features various extensibility points to make it even more useful.

Using the inspector extensibility mechanism, I quickly knocked up an inspector for typical federation messages (thanks for Eric Lawrence btw). Below is a screenshot for WS-Federation.

I also added support for SAML 2.0p request/response messages:

The inspector can be downloaded from the identitymodel Codeplex site. Simply copy the binary to the inspector folder in the Fiddler directory.