I've had that too. It happened on my commuter at the end of last summer, and most recently last weekend on my roadie. It was creaking with each hard pedal stroke on the right side, and also creaking once per wheel revolution when I had the right pedal at 6 o'clock with weight on it (e.g. when cornering to the left).

Both times it was the seatpost for me. Removed it, cleaned it, greased it and reinserted - creak gone.