Ask HN: Pick a language. What are some project ideas that show it off?

I realize that if I pick up a web framework, web projects are the obvious application space. But what about "raw" languages? If I want to see toy around with, say, haskell, what's a good application space to show off it's feature set/paradigm and build something that's not just a crappy "Hello World" application?<p>In other words, let's get a listing of project ideas (not startup ideas) per language that show off that language's strengths in a non-trivial but below expert-level way. Go!