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I found going through "Introduction to Algorithms" and porting the algorithms in the book to <new language> a great way to get used to a new language.

Okasaki's _Purely Functional Data Structures_ can be a good choice, too - depends on the language.

Write a game. ASCII Tetris or Pac Man or Space Invaders would be fine. Whether that is a simple afternoon activity or a somewhat epic undertaking might tell you something about the language.

I've been writing a couple of small games with Clojure + Swing as a way of learning Clojure, and it's been very smooth so far. I haven't even touched concurrency or macros yet but the basic language feels even more consistent and logical than Ruby.

Read some of the Clojure code, both in main and in contrib.