One thing I love about ruby is that mostly it is a very readable language (which is great for self-documenting code)
However, inspired by this question: Ruby Code ex
This slideshow is quite complete on the main Ruby idioms, as in:
Swap two values:
x, y = y, x
Parameters that, if not specified, take on some default value
def somemethod(x, y=nil)
Batches up extraneous parameters into an array
def substitute(re, str, *rest)
And so on...