OmniWeb has a few debugging features, it has the Error Log, which supports displaying http headers (Show HTTP requests) and OmniWeb also supports Drosera which is the debugger that used to be included with WebKit nightly builds but has since been integrated into WebKit itself. You can still grab it from older WebKit builds like this one though:
http://builds.nightly.webkit.org/fil...SVN-r33431.dmg