In a large saucepan, combine sugar and water and stir over medium heat until all the sugar is dissolved. Remove from heat and let cool slightly.
To the saucepan, stir in citrus zests and juices. Let stand, covered, for 1 hour.
Strain syrup from the saucepan and into a bottle that you can cover and refrigerate until cold.
To serve, fill glasses or a jug with equal amounts of fruit syrup and water. Add ice and enjoy!