Any wiki is basically an example, isn't it? Or if I'm not quite ready to defend that statement, I can at least say, you could use (or maybe misuse) any wiki-builder to build a Zettelkasten. Just impose a length restriction on each page's content.

But I don't know how much of the advantage of the Zettelkasten is tied to its being in the physical realm. On the one hand, the more physically rich an experience (of browsing or traversing or entering info for example), the more it will tend to stick in memory. On the other hand one of the things computers are good at is remembering indexes and linking them to other places where that index was mentioned.