Several utilities, such as Cocktail & Onyx, have the capability to clear the system cache.

But I am uncertain when and how often this should be done (if at all). I see no difference when 'tis done or not.

Is this something that one should do as (routine) preventative maintenance, how often?

Waddya think?

Personally, I only do it if I'm having a problem. Otherwise, as the saying goes, if it ain't broke don't fix it.

I agree with Jon about virtually all routine maintenance being in the status of "if it aint broke, don't fix it.". Over the years, the increased stability and functionality of OS X has rendered most routine maintenance unnecessary if not altogether obsolete. However, I do run regular checks of the volume and file structures as well as surface scans to provide early warning of impending system problems, but I do not consider these tests as maintenance as no files or data is changed by them.

Jon, What would be the characteristics of such a problem? Or, what kinds of things is it likely to fix?

In sum, I don't (even) know what conditions suggest it be done.

From OnyX Help:

System Cache
You can remove cache files in the System folders using the System tab of the Cleaning pane and the Automation pane. Any cache files used by the System can be removed without risk. You can recover disk space and prevent a corrupt file from impacting computer functions by deleting these files.

The computer can slow down temporarily after this operation is completed. This is normal, as the caches have to be rebuilt.

I can't get more specific, other than saying that I have cured a general system slowdown by cleaning caches. I agree with Joe that newer versions of OS X have been quite stable. I don't recall ever cleaning caches in 10.6.

Thanks all!

I think I've got it. I'll heed your wise words, and sweetness and light are sure to follow…

FWIW my take on when to clean caches is when Safari starts having otherwise inexplicable glitches that are not solved by restarting Safari.

System caches and Safari caches are different animals.

System caches include font caches, kernel extension caches, and so on. They can be flushed regularly, but usually there's no reason to. If there is a need to--for example, if you install a new kernel extension--the cache is flushed and rebuilt when the computer reboots.

If I recall correctly, rebooting with extensions off (holding down the SHIFT key during reboot) also flushes the system caches, which is part of the reason why it takes longer for the computer to finish booting.

the "purge" command used to be available, but I don't see it recently. (devtools maybe?)

It would grind the computer to an absolute halt while it ran, but you'd see a huge part of the yellow and even a bit of the red clean up in Activity Monitor. Saves a reboot a lot of the time to reclaim leaks etc.

Is there an alternative available currently? I'm running 10.6.8 (yes really)
purge runs right now but it errors out on me

apple:~ virtual1 $ purge
WARNING: Could not determine processor information.
WARNING: Could not determine memory controller information.
WARNING: Could not determine Operating System information.

(sudo doesn't help)

Running "purge" on my 10.6.8 yields:

Artie's-MacBook-Pro:~ artie$ purge
-bash: purge: command not found.

Did I do something wrong?

No problem running purge here in Mountain Lion.

Edit: it's at /usr/bin/purge