Clojure:

    (reduce + (range 1000000001))

Written that way, though, it takes a long time (108 sec, compared to 2.6 sec in C). So that's not fast, as elegant as it may be.

This is what I have for C. It's probably not great C code.

    int main() {
      long res = 0;

      int i;
      for (i = 0; i <= 1000000000; i++) {
        res += i;
      }

      printf("%ld\n", res);
      return 0;
    }

Faster than (naive?) C is this Clojure loop (no range object).

    user=> (time (loop [i 0 res 0] 
              (if (> i 1000000000) 
              res
              (recur (inc i) (+ res i)))))
    "Elapsed time: 1612.094 msecs"
    500000000500000000