Have you looked at a snippet of code, given it a double-take, then thought "Woah, what is that?" I've found this happens for languages I haven't used at all or often, and those that I am familiar with. Rather than forget those useful or obscure features, I've begun to assemble them in a repository, DidYouKnow. It started with some great examples from Python, like list comprehension, went onto Perl and regular expressions, and naturally, C++, easily the winner of an important code quality measurement.
Rather than hope that these examples actually do what they say, they are implemented as unit tests, and are (hopefully) as simple to run as possible. This proved to be an excellent way of testing and growing my C# knowledge for the interview of my now new employer, too!