APL
This language is incredibly powerful and very terse, it will hurt your brain.
For starters it's tricky to use without a custom keyboard, or at least a keyboard overlay to show up all the obscure symbols it uses.
Then the language is of the vector/array-based paradigm and specialises towards complex linear algebra. The original version did not even have loop constructs, anything and everything done by chaining rather unusual array operators together.
strip_tags()anybody? (borrowed from Wikipedia)
