Fibonacci numbers

*:'(+\|:)\[20;1 1]

This (in my eyes beautiful) program calculates the first 20 fibonacci numbers. It's actually rather simple. The +\|: operator is composed out of reversion (|:) followed by "traced" addition (+\):

(a,b) -> (b,a) -> (b,a+b)
      |:       +\

Now we apply this operator 20 times to 1 1, meanwhile tracing it: (\[20;1 1])

1 1 -> 1 1 -> 1 2
1 2 -> 2 1 -> 2 3
2 3 -> 3 2 -> 3 5
...

The first column is returned as a list. Now we just need to remove anything but the first element in each "sublist", that's exactly what *:' does.