My visual diff tool of choice is Meld, and most of us at work use it often with git. However, it would be cool if it supported plugins for filetypes. For example, recently I was interested in an SVG visual diff, which colours differences between SVG files, but could not find one.
But I take your point that professional designers tend to pay for their tools, whereas programmers often find an open source tool. I guess it comes from the fact that programmers can program their own tools, where as designers cannot.
I recall a project hitting the front page which focused on visual diffs of 3D model which you may be interested in [1].
[0] - http://xkcd.com/1110/
Thanks for the 3D differ link! The conventional wisdom when I talk to people about DiffPlug is that I should make it into a web service. For diffing, I think a desktop app is better than a web service for a few different reasons, but one of them is that the cloud is a pain for big documents like CAD files. I haven't had any real data to support that so far, but the comments thread for that link is very helpful!