If your drive begins to get too full, you have a few options. The simplest solution is to buy an external hard drive. With a laptop, you'll want to move larger or infrequently used files to the external drive; with a desktop system you can simply make it a second hard drive for regular use. You can also replace the internal hard drive of either a desktop or portable Mac with a larger drive.
Finally, you could try to trim the amount of data on your current drive. Disk Inventory X (free) and id-design's WhatSize (shareware, US$13) are two helpful tools for discovering just what files are eating up hard drive space and which can be easily moved to a secondary drive or potentially deleted altogether. Prosoft's US$99 Drive Genius, which offers several hard drive tools in a single application, now includes a DriveSlim feature that can search for large files, files that have not been accessed recently and duplicate files.
Note: This same rule applies to the embedded version of Mac OS X that powers both the iPhone and the iPod Touch. If you notice erratic behavior on those devices and they are filled to bursting, removing some content may help (though there could be other causes as well).
4. Delete cache files
Many applications rely on cache files to improve performance. The most obvious example is Web browsers, which cache images and other content from Web servers to speed up repeated access to the same files. Leopard itself maintains a series of cache files for improving system performance when using a number of features.
Cache files can present problems if they become corrupted or damaged. The operating system or an application that relies on the cached data may behave erratically or crash because it can't properly read the data in the file -- leading to potentially more corruption if an app crashes while it's writing to the file.
Unlike files in the Unix /tmp directory, cache files aren't cleared when a Mac is rebooted, which means that even when they aren't corrupted, cache files can sit around taking up space on your hard drive long after a given application is deleted. They can also retain settings and private information that you may wish to get rid of. As such, pruning cache files is a prudent choice, particularly if you notice that an application isn't as stable as it used to be.