If you regularly use your Mac (or any computer, for that matter), then you will undoubtedly run into a problem or two. For the most part, issues are small configuration hiccups that can be fixed by at the very least restarting affected programs, logging out of your system, or restarting it. However, there are times when a problem may require more attention in order to properly address.
In these cases, the use of various system utilities to monitor your computer's activity can help pinpoint problematic connections, processes, or other aspects of your system that may be contributing to the issue at hand. Apple includes several handy utilities in OS X, such as Activity Monitor and the Terminal, but in addition you can use several third-party tools to greatly increase your ability to troubleshoot problems and fix errors.
The first tool is a good text editor. Apple's TextEdit can be quite useful, but it has some limitations, especially if you plan on reading scripts and configuration files throughout the system. While there are a number of third-party text editors available, one common, powerful, and free one is TextWrangler by Bare Bones software.
In addition to a text editor, a data rescue tool such as Data Rescue from Prosoft Engineering can be exceptionally useful for recovering inadvertently deleted files. While backup solutions like Time Machine are a recommended first choice, if you have an external drive that fails and is not backed up, then software that scours it for remnants of file structures can show promising results.
A good drive testing and recovery package may also be useful. Apple includes Disk Utility and the command-line "fsck" options for managing filesystem structure. You can use a tool like Alsoft's Disk Warrior, which will help rebuild a corrupted disk, and which may succeed with Disk Utility fails.
Since many computer services use the internet, you might consider an option for monitoring connections and bandwidth, such as the reverse firewall Little Snitch by Objective Development. In addition, you can go as far as installing a packet analyzer tool like WireShark to finely detail where connections are being made, though the sheer detail from this program can be daunting for novice users.
The next utilities help you monitor disk usage. While the Finder and other built-in services in OS X can easily show you the space used on disk, it does not readily show what specific files and folders are contributing to the space used. To get this, you can use the free tool GrandPerspective, which will scan a given folder and show file sizes in a colored grid, making identifying large files very easy to do.
Finally, as the system and programs run, they regularly write to files on disk. If for some reason you are seeing a file or two appear in an odd place, or otherwise suspect one is being changed when you perform a given task, then you can use a filesystem events monitoring tool like Fern Lightning's "fseventer" to graphically show the filesystem modifications that occur as the system runs.
These tools will allow you to monitor your system to a greater extent than if you only use Apple's included utilities, and can give you an edge up on tackling problems you may be experiencing. Even if you are not seeing any major problem, these tools can be useful for generally understanding your system better.
Do you have any utility recommendations for helping troubleshooting in OS X? If so, then let us know in the comments.