Although commercial spot carpet cleaners will remove many carpet stains, here's a handy DIY recipe for one.

General stain remover
Stir one teaspoon of liquid dish detergent into one litre of warm water. Add 1/4 teaspoon of white vinegar. Let the solution sit on stain for 10 minutes before blotting up.

White vinegar solution
Mix one cup of white vinegar with two cups of water.

Use on alcohol and soft drink spills; coffee tea, urine and vomit.

Source: Better Homes and Gardens Making a Home (Meredith Publications)