Math as Code
github.com
github.com
Probably more useful for mathematicians implementing their papers than the other way around, but I find it kind of a nifty language feature and makes it easier to verify that your program matches the math.
Haskell has this feature. While it is easily abused, it lets you write functions like:
a ∈ [] = False
a ∈ (x:xs) = a == x || a ∈ xs
(some might consider this abuse)