Kronk is a cross-platform Ruby gem that works much like a combination of curl and diff, bundled with basic data parsers (JSON, XML, PLIST).
Kronk also includes a robust data traversing idiom to isolate and compare the returned data so you can look at and/or diff specific data points in the returned data.
Finally, Kronk is highly customizable with support for URI-specific configuration, and custom data parsers.