Ask HN: Cool Demo Project Ideas

The standard (and probably true) wisdom around here is that a good way to get noticed and get the kind of job that most of us would like is to "build something".

What I'm hoping for is ideas for demo projects. I don't need anything designed to make money or steal anyone's startup idea, just things that are fun to build and show development skills.

Demos you've seen already and liked or that have gotten people job offers are also appreciated.