James Delaney’s created The Unfocused Brain, an interesting variation on Mitch Altman’s Brain Machine:

This program and schematic is designed to flash LED’s in certain brain wave inducing patterns. The device is controlled by four buttons that switch the generator between Alpha, Beta, Delta, and Theta frequencies.

After building and trying out the SLM I noticed that certain transitions from one state to another created particularly strong visual hallucinations.

My project is a deviation from the original SLM. Since my hallucinations seemed to be the same with or without the audio component I decided to leave it out. I added buttons rather than a program so the user can change frequencies spontaneously. I built this project with the Attiny461 chip.

Check out the Make Brain Machine thread for more ideas on variations.

And, if you get really ambitious, switch to low-frequency ultrasound for brain stimulation (pdf; thanks to Bryan Bishop for the link)