I agree with Alki that frag plugs are ugly and IMHO the best looking and best bang for you $'s is 1x1 or 2x2 tumbled marble/travertine/limestone tiles you can pick up at HD or Lowes for $7-10 s/f (144 ea 1x1's / 36 ea 2x2's). They are natural, easy to cut/shape and blend into aquascape much better than a round disc/plug.
Todd