This is absolutely – well, very little – to do with outlining as such, but it's such a wonderfully clever and freeform tool I thought I'd mention it anyway.

I've recently been playing with Sparkle, a web design editor that is completely freeform, but has some very clever features. It's still only in its early development stages, but shows enormous promise.

If you're looking for a user-friendly, highly flexible WYSIWYG web editor for Mac, I can very strongly recommend it. The developer is very nice, too!