Mostly water. About 98% water, in fact.
Saliva also contains dissolved salts, mucus, some antibacterial molecules, and enzymes such as amylase, which breaks down starch into simple sugars. Another enzyme, lipase, breaks down fat. So saliva is actually the first step in the digestive process.
Saliva in mice includes a hormone called nerve growth factor, which helps healing when the animals lick their wounds. It is not a component of human saliva, however.
In humans, antibacterial agents such as peroxidase, immunoglobulins, lysozyme (an enzyme that breaks open bacterial cell walls), and lactoferrin (a protein that kills bacteria and fungi) are produced in saliva.