I was just going to report that I tested some online and off-line (*.exe) diff tools again. Thereby, the installed tools, like the latest versions of WinMerge and Meld, proved to have no chance against the best online tools. Both WinMerge and Meld fail completely as soon as the texts become just a little dissimilar.
I checked like a dozen online comparison tools, and the best ones (by far) still seem to be Vroniplag
(great multi-color highlighting of identical textblocks including moved block detection), and wikEd diff
(highly configurable, including moved block detection). Vroniplag even lets you copy the results including all the color highlighting into Word, however only if the comparison is done with Internet Explorer. Both with Chrome and Firefox, this does not seem to work properly.Copyleaks.com
seems to be even more sophisticated since it at least tries to detect similarity also when there are small differences like for example OCR errors. Also, you can click on text passages and it will highlight the respective passage in the other text. However, it is a) terribly slow as compared to the other two mentioned above, b) has no multi-color coding for quick overview, c) breaks longer texts into multiple pages that you have to click through, and d) you can't quickly edit one or both of your texts and redo the comparison with the edited texts.
I am yet to discover a service or tool that uses some kind of fuzzy logic or artificial intelligence in order to spot and visibly highlight similarities in a "stepless" manner, for example by using some sort of heat map color highlighting of the similarities.