Even that pales next to the storage capacities of external hard drives, which are preferable for really large data dumps like copying an entire iTunes library prior to pruning it back, for example, just in case you delete something you didn't mean to. Or, for backing up items like large Photoshop, music or video projects where accessing the data from an optical disc can be painfully slow.
Get a cheap external drive
USB 2.0 and FireWire hard drives are now so big and so cheap that they can conceivably be both a Time Machine backup drive and a general purpose archiving hard drive at the same time.
A quick search reveals several 1TB models for around £100. At ten pence per gigabyte that's incredibly cheap. At these prices you can have a 500GB or 1TB external drive connected to your Mac and just use it to store all non-essential items, keeping your boot drive nice and lean.
As we have seen, media files, emails and documents can be sorted, backed up and then deleted easily, and certainly this will reclaim a lot of space back on your system. Under the hood, however, there's a lot of activity that isn't directly visible to the user, and which can result in additional clutter. Having a huge iTunes or iPhoto library may slow down their launching times, but clogging at the system level can affect OS X itself negatively in terms of speed and stability.
In order to function properly, the system must write files to the hard drive all the time. These files take various forms – temporary files, logs, caches and more. In the short term, they can help with remembering settings and recent tasks.
In the longer term, they can start to occupy more space than you would ideally like. Curiously, OS X doesn't have a built-in tool to clear out these files, and most people aren't comfortable with using Terminal to do it.
Help is at hand, though, in the form of various third-party utilities, the best of which is OnyX. Best not just because of its design, but because it's free and on this issue's disc.
Clean up with OnyX
If you download the correct version for your flavour of OS X, you can use its Cleaning section to dig out all the system stuff that has built up, much of which will be obsolete.
The Internet tab lets you delete all web browser history, caches and form values. Caches helps you to clear out any troublesome cache files that may have become corrupted. Logs will remove the many small text files that OS X writes as actions like crashing or software installation occur.
Finally Trash lets you empty the main Trash, and in the Misc tab you'll find the option to delete things like downloaded Mail attachments and previous iTunes libraries. After these it's recommended to restart, and if you monitor the size of the hard drive you'll find that there is more free space after restarting. How much exactly will depend on how much clutter was on your system.
There are some other good tips for reclaiming space. If you have had your Mac from new and it came with iLife preinstalled, you will almost certainly have the GarageBand library and iDVD themes occupying a few GBs of space. If you don't want to use the content that comes with these programs, delete them.
They can be reinstalled later from the iLife DVD if you change your mind. GarageBand's loops can be found in User > Your Username > Library > Audio > Apple Loops and iDVD's themes in User > Your Username > Library > Application Support > iDVD > Installed Themes.