Linux kernel 2.6.16 and later has a option which let's you free inode/dentry and pagecache caches on your system. The file is located on /proc filesystem.
It has 3 options. For example:
# echo 1 > /proc/sys/vm/drop_caches
frees pagecache on your system.
# echo 2 > /proc/sys/vm/drop_caches
frees inodes and dentry caches
# echo 3 > /proc/sys/vm/drop_caches
frees pagecache and inode/dentry caches.
tip: some dirty caches are not freeable so it's better to use "sync" command before using drop_caches for freeing much more memory on your linux system.