Fiddler
This post by James Clarke reminded me that I should mention the excellent Fiddler HTTP debugging proxy, not least because it is a very useful tool for viewing XML-RPC requests and responses.
Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler is designed to be much simpler than using NetMon or Achilles, and includes a simple but powerful JScript.NET event-based scripting subsystem.
Fiddler ships with several request and response inspectors installed by default. The following snapshots show the output for an XML-RPC request.
Request Headers Inspector:
Request TextView Inspector:
Request HexView Inspector:
Request Raw Inspector:
Request XML Inspector:
There are a couple of articles on MSDN which provide a comprehensive introduction to what you can do with Fiddler: HTTP Debugging and HTTP Performance.