You can synchronize your brainwaves to that of an external stimulus like sound, light, and even electromagnetic radiation in order to easily induce brain states like sleep for example. Think of it as assisted meditation which is effective at treating conditions like ADD, insomnia, and much more. Gnaural is brainwave entrainment software which generates binaural beats. It is no longer in the repositories and it doesn’t have a PPA, but 32-bit users can download and install the .deb from the website while 64-bit users like myself are forced to compile.
http://gnaural.sourceforge.net/download/
via The Silent Number: Top things to do after installing Ubuntu Linux 9.10 Karmic Koala.
