The `purge` command has a pretty short man page:
> purge -- force disk cache to be purged (flushed and emptied)
> Purge can be used to approximate initial boot conditions with a cold disk buffer cache for performance analysis. It does not affect anonymous memory that has been allocated through malloc, vm_allocate, etc.
[This seems to talk about it more: http://workstuff.tumblr.com/post/19036310553/two-things-that...
On my 10.5 laptop it didn't seem to dramatically decrease the memory marked "inactive".]
I have regularly seen 1-3G of ram get free'd up on a 'purge'