A long while ago I made a Java-based tool called k/stencil, though (unusually for me) I somehow removed it from its place on my website, and can't find the old code.
In short I could use it to make stuff like this:
Anyway. These days, CSS can totally do the same effect. To whit:
background-size: 400px 300px;
can make something like
So, to honor, or replace, my old tool, I thought I'd build a tool. But then I realized that was kind of overkill... I mean the tool I was envisioning would be a highly configurable expert-ish tool, basically saying "ok, here's the basic CSS, you edit it and I'll show you the changes updating on a sample!". Which is exactly what jsFiddle is. Duh.
So I made one of those!
UPDATE: I changed my mind, because why the jsFiddle is ok, it doesn't let you quickly play with sliding the image or changing it's zoom, so I reinstated http://kirk.is/features/kstencil/. I used basic jQuery mouse events for sliding the background and used noUiSlider as a lightweight library for the zoom widget. Anyway, check it out, it's rough but effective.