Windows only: Paragon Partition Manager is a feature-rich application for partitioning your hard drive, optimizing your disks, creating simple partition backups, and more. It's normally $40, but today you can snag it for free.
We'd almost never suggest shelling out cash for an application whose sole purpose is partitioning, especially when an application like the previously mentioned GParted Live CD is perfect for partitioning and completely free, but if you need more from your partition manager, commercial apps like Paragon Partition Manager can be useful.
We haven't tested it thoroughly, but Paragon promises an impressive feature set, and—for today at least—you can't really beat the price. If you've used this app in the past, share your experience in the comments.
Partition Manager 10 [Paragon Software]