It's used in some central systems, as in some of the ascending reticular activating system that is involved in, among other things, the sleep-wake cycle. Glutamate is a very important excitatory neurotransmitter at central synapses. It's particularly notable in the retina and in visual cortex. Enkephalins and endorphins are peptides that affect some central synapses and allegedly explain the pleasant effects of exercise and accupuncture. I prefer the TV couch and to remain unpunctured, however.
Dopamine is a very important transmitter, particularly in central motor function.
Too much dopamine is often associated with behavioural hyperexcitability, as in Gilles de la Tourette's syndrome, in which some sufferers sing and swear loudly in public and twitch. Dopamine is also likely to be involved in controlled sequential processing in the brain, as disorders of dopamine metabolism are often associated with schizophrenia - in which the orderly sequential progression of mental images or states becomes chaotic. Serotonin is also likely to be involved in limbic function - because it is a property of the brains of suicides that they have low serotonin levels. Some drugs for depressiona and anxiety also seem to affect serotonin metabolism. Finally, but importantly for all you music lovers, Ecstacy is presently thought to affect serotonin metablism.