I'd like to know what current tools there are for doing HTTP compliance
testing. At this point, I don't care whether they're free or not. I don't
care whether they use Java or something else.

I'm familiar with the Java HttpUnit package, but that's currently focused
more on HTML testing, although it mostly doesn't get in the way of "manual"
HTTP compliance testing.

There are probably other tools like HttpUnit, that just present an API where
you can send a request and process the response. A more fully-functional
HTTP compliance testing tool might have two basic features in addition to
this:

1. Being able to control not only the request being sent, but the resource
being retrieved by the server. For instance, to build detailed tests of the
"If-Modified-Since" header. Tools like HttpUnit don't get in the way of
this, you'd just have to build that architecture yourself.

2. Intrinsic, or even "rule-based" knowledge of the rules of the HTTP
protocol, without having to hand-code everything.

This is probably wishful thinking, but it can't hurt to ask.