In generic terms, you probably want to look at Gparted which is a disk partition tool similar to Partition Magic. It will be available in the add/remove if it's not installed by default. When I run it from my program menu as a regular user, it prompts me for a password to run it as Root. (This may be a Debian 6 specific setting but I would expect Ubuntu to do something similar if not the same)
Caveat; your working with file partitions here and *nix does not like to work with mounted file partitions since you never know what other users may be working with that partition. Gparted and similar tools do not like to work with mounted partitions.
I think what you actually want to do is get the Parted Magic or Gparted liveCD. Boot from that, it'll load into a GUI desktop with a gparted icon or it'll auto-start gparted for you. Since your running from the liveCD rather than what's installed on the hard drive, you can mess with partitions all you like.
Remember that with any OS, messing with partitions is asking for something to break. Be sure you have backups of anything you can't reinstall.
If you want to work from the command line specifically then look at the basic Fdisk or the more complex Parted tool. Parted is the tools that works behind the Gparted GUI front end and Fdisk is the old-school tool.
To be honest, I liked Mandriva's partition tool enough that for years I'd simply boot the Mandriva Free install DVD and walk through it until the partition manager. Once the HDD was setup, I'd cancel out of the Mandriva install and start over with whatever OS I intended to install on those freshly created partitions.
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