The market leader is Symantec's Partition Magic (formerly a Powerquest product, before Symantec bought them out.) My favorite is either of 7Tools Partition Manager (it looks like a Partition Magic knock off, but underneath the covers, it is more solid) or GNU's parted.
Typical installers for major distributions, likely including Ubuntu (though I don't know for sure) include a variation of parted that you can bring up during the install, if you choose the Advanced Partitioning or some such option at the point it comes to decide where to lay down the installation on your disk(s).
That's why I put my caveat at the end. Partitioning always made me nervous, so I always did a complete backup and just formated and partitioned. That was a few years ago, and the disk utilities then seemed to be more like hatchets than scalpels. They also worked from a dos prompt and had warnings with exclamation points.