Concurrent prime number sieve. I wanted something that took a while, so that the agent could be running in the background for some time. I used an atom so that I could monitor the sieve externally.

concurrent_sieve.clj

(def sieve (atom []))

(def siever (agent sieve))

(send-off siever
  (fn [x test-set]
    (doall
      (map (fn [n]
        (if (not (some
                    #(zero? (mod n %))
                    @x))
            (swap! x (partial cons n))))
        test-set))
    x)
  (range 2 30000))

(dotimes [n 100] ((prn (count @sieve)) (Thread/sleep 500)))