A sulfur-containing amino acid. Its sulfhydril group is very effective at binding free radicals and prevent their toxic effects. It is not one of the essential amino acids, but blood levels can easily be raised above baseline by supplementation. Cysteine's antioxidant function is similar to that of glutathinone, but cysteine is less expensive and more bioavailable.

Cysteins is particularly effective at preventing the cross-linking damage done by ethanol's toxic metabolite, acetaldehyde. Many sources recommend taking up to several grams of cysteine while drinking to prevent hangover (my personal experience supports the idea).

The biologically active isomer of cysteine is L-cysteine. It is often sold in the form of N-acetyl-cysteine, or NAC, which is a more stable molecule.
Information is from an article by Dr. Steven Fowkes at http://www.ceri.com/alcohol.htm, though I've confirmed much of it elsewhere.