Stumbled over this wonderful commit to the tree earlier today.

In short, it allows you to dump pagecache/dentries/inodes for profiling stuff in the system. It's present in kernels newer than 2.6.16.

To free pagecache:
 echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
 echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
 echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.