Drive Genius is a hard-working hard-drive utility from longtime Mac developers Prosoft Engineering, with the honorable distinction of being employed at the Apple Store Genius Bar as part of their ProCare Yearly Tune Up service.
The interface of Drive Genius shares some similarities with that of its data-recovery sister program Data Rescue: you navigate through a 3D "tool arena," a sort of virtual lazy Susan of hard-drive tasks that's fast and fluid enough to not be annoying (and it's actually kind of fun). You choose from six primary tasks, Information (with detailed readouts on S.M.A.R.T. status, file system type, and more), Defrag (to optimize drive speed), DriveSlim (to free up space by eliminating duplicate or unused files), Repair (to verify, repair, or rebuild a volume), Scan (to check for bad blocks), and DrivePulse (a monitoring tool that checks your drive in the background, when your computer is idle--although unfortunately there's no way to turn off its menubar icon if you decide to use it). You can also flip to the other side of the "tool arena" for even more esoteric tasks: Integrity Check, Initialize, Repartition, Duplicate, Shred (with up to 35-pass Gutmann erasing), Benchtest, and (perhaps scariest of all, "for expert use only") Sector Edit.
Many of these tasks can be time-consuming, but Drive Genius does a good job of keeping you entertained with graphs, lists, and live updates--or you can always just have Drive Genius notify you by email whenever it completes a task. You'll also want the DVD version of this program (which you can either have mailed to you, or download and burn yourself), so that you can work on your drive when it's not running the OS (or when it won't boot at all).
Drive utilities aren't for everyone (and Drive Genius isn't cheap), but if you're interested in keeping your Mac in tip-top shape, it's hard to go wrong with a tool that the pros use. Drive Genius gives you a lot of features in an all-in-one package that's easy to use and understand.