How has Ruby blown my mind?

In a word iterators. No single language feature has changed the way I think about code more than the simple, now seemingly obvious, pervasive, compelling act of passing anonymous code into a method to be run in context by that object. In a few days, this feature became so natural to me that languages where it was missing - Java, C, Perl - were harder just because I knew it was missing.

By the way, if you don't know about Pat Eyler's & Apress' ongoing blog about Ruby contests, check out this month's contest.