There is some FOSS alternative if I recall correctly but I forgot the name. I think it was written in Java.
The Cocoa-specific app looks interesting, but I think the Java-based alternative you were referencing was Paros[1]?
CocoaRestClient? http://code.google.com/p/cocoa-rest-client/
Paros?
[1]: http://www.charlesproxy.com/