"His initial investigations involved determining how much activation at specific sites in the brain was required to trigger artificial somatic sensations, relying on routine psychophysical procedures. This work soon crossed into an investigation into human consciousness; his most famous experiment demonstrates that the unconscious electrical processes in the brain called Bereitschaftespotential (or readiness potential) discovered by Lüder Deeckean and Hans Helmut Kornhuber in 1964, precedes conscious decisions to perform volitional, spontaneous acts, implying that unconscious neuronal processes precede and potentially cause volitional acts which are retrospectively felt to be consciously motivated by the subject."
http://en.wikipedia.org/wiki/Benjamin_Libet
That's a mouthful. You can read about the details of the experiment at the link.
It would appear that some kind of neuronal activity precedes what feels to us like a free decision, by some predictable fraction of a second. We do not experience this event in our consciousness. We do experience the making of a decision freely later than the unconscious neuronal event. It looks very much like the neuronal activity causes the next series of events, which we experience as a free choice. The actual experimental work was published at:
Libet, B., Gleason, C. A., Wright, E. W., and Pearl, D. K. (1983). Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act. Brain, 106:623-642
The results have been confirmed, and Libet has been recognized and honored for his work. Although he is well known in his field and some related fields, very few lay people know about this work. The implications of the work have been explored by distinguished thinkers and writers. Start at the Wikipedia link if you are interested.
I find it astonishing that there is concrete evidence that the experience of free will is a delusion. I find it more astonishing that it isn't front page news.