sweetmactips:

Leopard can re-size partitions on the fly! If you need to add another partition (treated like another hard drive, but still part of the same physical disk) Leopard can do it without losing anything currently on your computer. It’s very useful if you need a new test environment and don’t want to lose what you’ve got done already. Just select your disk, go to the Partition tab and click the + button. Cool!