I feel like these aren't very "hidden", yet any time I talk about these or their developers to my colleagues, nobody knows what I'm on about.
Crystal's Ruby syntax is a con for me. Pony could be great but the community is being mismanaged terribly, it's a real shame. I think Julia is suffering the same fate as D for general programming. It's got a lot going for it but it's not interesting enough except for certain niches, and hasn't gained enough momentum.