If you use your dishwasher a lot, it can easily get gunked up with leftover food, grease, and soap scum. This leads to an appliance that smells more like a sewer than a cleaning device. On top of that, the dishes that come out of it might not be as sanitary as they should.
Step-by-Step
Remove and clean the filter. Soak in warm, soapy water for 10 minutes, then replace.
Pour 1 cup of white vinegar into bottom of empty dishwasher. Run a heavy clean cycle.
Sprinkle 1 cup of baking soda in the bottom of the dishwasher and leave overnight. In the morning, run an empty cycle.
Gently scrub any remaining dirty areas with a toothbrush.