HTML Diff Tools?

Anyone know of good HTML diff tools that actually work well? For that matter, does anyone know of any diff tools that work well with plain text that isn’t line based, eg: the standard type of text you’d find in a blog entry or a book? I’m guessing a combination of word based and line based diffing might work okay for that type of thing but I haven’t come across much that actually tries to deal with the problem. At least, not in a way that aims to provide a meaningful result for humans rather than just a form of compression for updates.

I can probably wrangle the HTML side of it well enough if I had an existing tool that could take the plain text and provide meaningful differences.