There are a variety of tools available for managing your hard drive's structure and data. Most are shareware utilities that give you function or time limits until you register the program, but there are a few out there that are both useful and free.
It's a free utility that scans all files starting from a target directory and displays them in a proportionally sized grid based on size. It is a convenient and intuitive way to see what directories and files are taking up space on your hard drive, and I've used it quite successfully for years.
An alternative to Grand Perspective, this program gives an overview of the space usage on the disk and allows you to remove fils you do not use anymore. It offers the same functionality through a different interface, so you have the option between this and Grand Perspective for what best agrees with you.
A free check for the SMART status of your hard drive, which is used to predict drive failure based on drive behavior. The system checks this at boot, but it may be a long time before the SMART status is checked again for systems that are kept running. SMARTReporter regularly scans the drive for evidence of failure.
This is one very useful program that I recommend all Mac users install on systems running 10.5 or earlier. Unfortunately, the utility has not been updated to be fully compatible with Snow Leopard, so 10.6 users should hold off. Like OnyX, Cocktail, and other similar maintenance utilities, Applejack runs permissions repairs, disk repairs, and clears cache files, but unlike ones that run when logged in Applejack runs in a minimalistic environment (when booted into Single-user mode), which prevents interference by other applications and system processes that may be running at the same time.
For more routine maintenance tasks, OnyX may be more convenient than Applejack. You can run it when logged in and more easily manage details of which caches and temporary files to remove or rebuild. There are other cache cleaners out there, but some are not free.
NOTE: These are not utilities to fix drive structure problems such as partition troubles. For that we highly recommend you use a highly-developed and supported solution such as one of the following utilities:
These utilities are best for identifying bad blocks, managing partitions, defragmenting and optimizing, and direct sector editing among other features.
Have any suggestions? Lets hear about them!