Brian France pointed this out to me, it's useful stuff, particularly when benchmarking I/O.

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

It's a good idea to sync first, to assure that all dirty page caches are dropped.