free

Do this only if you know what you are doing!

Switch to Super User

:$ sudo su

To free pagecache

:# sync; echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes

:# sync; echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

:# sync; echo 3 > /proc/sys/vm/drop_caches