I don't know about you, but a partition editor has been part of my computer support tool kit for a long time. All modern OSs include basic tools that will let you format and partition a volume. This isn't enough for me, though. I like having a partition tool that supports nondestructive partition editing, and this usually means that I have to resort to third-party partition tools.
Windows users have a number of options if they're looking for a more robust tool than the Disk Management app that Microsoft provides. PartitionMagic is my nondestructive partition editor of choice for Windows, but there are also a lot of open-source options developed by the Linux community to support the partitioning of Windows/Linux dual-boot systems.
Mac OS X's default tool for volume management—Disk Utility—is also pretty basic, providing a simple means to format storage media. Apple doesn't want to make it easy for users to screw up their system partitions. The partition management landscape has changed a little in the newest version of the Mac OS, since support for dual-booting with Windows is now bundled aboard. Boot Camp, the application that makes this dual-booting easy on Macs supports nondestructive partition resizing, but you can't access the resize function outside the Boot Camp set-up app without delving into the Mac OS command line. That's too difficult for all but the geekiest (and the bravest) of Mac users.
Fear not, however, if you have a Mac and want to start doing some advanced monkeying with your partition schemes. iPartition is a must-have for any tech who needs to support Macs, especially in a cross-platform environment. It offers a lot of the features I love in PartitionMagic: nondestructive resizing of both Mac and Windows partitions, and even conversion between partition tables of different types (Linux partition support is a future possibility). iPartition is very user-friendly and even includes a companion application for defragmenting your drive before you start creating new partitions; that app is called iDefrag.
Don't let the vaguely annoying perpetuation of Apple-related "i" naming conventions put you off using iPartition. I've used it several times as I've been setting up my Macs to dual-boot Linux, and it's worked great. I also have it on good authority that Apple support refers people to these products when users find that Boot Camp's built-in partition resize tool won't work on their system. For anyone who needs an advanced partition management tool for their Mac, I don't know of anything better.
P.S. I shouldn't have to remind any of you of this, but before touching your partitions, make sure your data is secure. Nondestructive editing isn't intended to stand in place of a backup, it just saves you from the drawn-out wipe and reinstall cycle that came with making these changes the old-fashioned way. Editing partitions without a backup is just dumb. Don't be dumb.