This Mailing list post from a user of “MythTV” reminded me of the wonders of LVM (Logical Volume Management) which is built in to Linux (HowTo, Resource page). I first saw LVM years ago on a commercial Unix, and didn’t quite understand the point. Now I see the point clearly, and set up most new machines with LVM.
If you’re not using LVM, and you have non-trivial storage hardware (more than one hard drive), now is the time to start.
On an OS without LVM built in (as I am, on my Windows machines..), it is often necessary to do things offline, with external tools (like Partition Magic) that could be done online in a running system with LVM.