A second best would be some ready-made package of files and accompanying howto for setting up such a server locally (ideally it would be possible to distribute this "test server package" along with one's module, so that users of the module had the option of adding the capability of testing more communication conditions using their own servers).

There's also the "Potemkin solution": use something like Test::MockObject to create a false interface to, say, LWP::UserAgent, and have it generate mock user agent objects returning mock HTTP response objects, for example. This alternative is least desirable because it could only produce a test suite that was as good as the author's grasp of the protocol(s) involved, but, arguably, such grasp is, at bottom, one of the things an ideal test suite would test (assuming that the code and the tests are authored by the same person, as is often the case).

My question to you, fellow monks, is what "fixture" servers do you use when you want to test internet client code? In particular, how do you ensure adequate coverage of those parts of your code designed to check for and handle anomalies in the communication? How do you make these tests portable?