Fractran is a cool esolang that looks impossible to write any programs in at first until you systematically trace through some examples. A while ago I wrote an assembler in Haskell with an optimizer so you can write programs in assembly-style.

  λ> pretty (sumTo 10)
  c += 0
  n += 10
  while n >= 0 {
    c += n
    n -= 1
  }
  λ> runAssembler (sumTo 10)                      -- Program length: 31
  Right [847425747 % 2,13 % 3,19 % 13,11 % 3,11 % 29,31 % 11,41 % 31,23
  % 11,23 % 47,2279 % 23,59 % 301,59 % 41,67 % 413,329 % 67,61 % 59,73 %
  61,83 % 73,71 % 61,71 % 97,445 % 71,707 % 89,103 % 5353,103 % 83,109 %
  5459,5141 % 109,107 % 103,113 % 749,113 % 19,131 % 113,29 % 131,127 %
  113]
  λ> runAsm (sumTo 10)
  [(Prime 97,55),(Prime 107,1)]

Just whipped up an interpreter in Racket for this, returning values lazily in a stream.

Figuring out how to get actual prime numbers out of the primegame program was the hard part.