Which other programming languages apart from Python and predecessor are out there using indentation to define code blocks?
Wikipedia has an extensive list of languages that use the off-side rule1:


  
  ABC
  Boo
  BuddyScript
  Cobra
  CoffeeScript
  Converge
  Curry
  Elixir (, do: blocks)
  Elm
  F# (if #light "off" is not specified)
  Genie
  Haskell (only for where, let, do, or case ... of clauses when braces are omitted)
  Inform 7
  ISWIM, the abstract language that introduced the rule
  LiveScript
  Miranda
  Nemerle
  Nim
  occam
  PROMAL
  Python
  Scheme, when using e.g. SRFI 119
  Spin
  XL
  


1: I've never heard this term before myself.