I would have said 'little-known' or 'not as well-known as they deserve to be', but it's a fair point. Many of them are not really hidden so much as underused.

I would have said 'little-known' or 'not as well-known as they deserve to be', but it's a fair point. Many of them are not really hidden so much as underused. (As I recall all of the "Hidden features of language X" questions on SO tended to drift that way pretty quickly.