Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
If you want to add space to a drive, store personal data on a separate partition from your operating system, or run multiple operating systems from the same hard drive, then you need a partition editor.
Traditionally, this is a field where alternative operating systems have always lead in the available tools. PartitionMagic, the first partition editor, started life as an OS/2 program for users who wanted to dual boot, and GNU Parted brought partition editing to the GNU/Linux command line in 1999. Since then, various interfaces -- most notably, GParted -- have tried to bring GNU Parted's functionality to the desktop.
However, from what I've seen, by far the most successful of these efforts is the newly released PartedMagic version 3.0. The name suggests that PartedMagic is meant as a free alternative to proprietary tools like PartitionMagic -- a goal in which it easily succeeds, despite a mildly eccentric desktop.