In the past, changing the size of a Macintosh volume has been a significant undertaking, requiring third-party tools or offline reformatting. But, all that changes in Leopard. In the newest version of OS X, you can grow or shrink any HFS+ partition. The resizing is done live, and can even be performed on a mounted boot volume (though we wouldn’t recommend it).
For more details, read the post on Make Mac Work, our sister blog written for IT professionals who support Mac users:
Make Mac Work: Resize Disk Partitions.
Warning: This is the sternly-worded paragraph that reminds you to never resize a partition without a complete current backup. Got it?
Source: This tip comes directly from Jordan Bojar’s weekly Make Mac Work blog.