Probably the most prevalent of these is Bacillus thuringiensis, commonly known as Bt. This bacteria produces a spore that kills certain moth and caterpillar larvae by paralyzing their guts. There are several strains, each targeted to a different pest. I have only seen Bt in a powdered form that must be mixed with water and applied with a hose attachment. Beneficial parasites are another popular type of biological control, the most common being nematodes.
Beneficial nematodes are microscopic worms that are predatory to certain insect larva.
The nematode then feeds on the dead insect and reproduces rapidly. This form of biological control is also generally purchased in powdered form and applied with a spray attachment or injected directly into the soil. Attempts to discover more parasites and pathogens that will control garden pests continue. If the ones I have mentioned do not work for you, contact Cooperative Extension for other possibilities. If you are a serious gardener, you should also be reading at least one of the more reputable gardening magazines so that you have some sense of what is new to the market.