Lucid Dreaming

Lucid dreaming is dreaming while being aware that you are dreaming, and then actively control what you do. Learning how to be conscious in dreams is hardly new. Aristotle mentioned the phenomenon and Tibetan Buddhists have been practicing lucid dreaming as a tool for spiritual development for more than a millennium.

Some believe that there is not such a state as lucid dreaming. They do not deny that sometimes in our dreams we dream that we are aware that we are dreaming. What they deny is that there is special dream state called the 'lucid state.'

More information:

The Lucidity Institute
The Official Lucid Dreaming Appreciation Page
Brilliant Dreams
Lucid Dreaming Mask