In the question of a biological cannon How would a Biological Cannon work? , one option was to use biological by-products for chemical propulsion of the cannon ball. It might have also been propelled by a non-chemically reacting high pressure gas or fluid.
We can do all sorts of simple chemistry with biological materials, and get new compounds; for example using burnt wood ash to create lye. When yeast ferments fruit to create alcohol, there is an upper limit to the percentage of alcohol (15% to 25%, according to the good folks on the Alcohol Stack Exchange: https://alcohol.stackexchange.com/questions/110/what-is-the-highest-alcohol-content-achievable-through-brewing#120). If we distill the alcohol, of course, we can some decent fuel. The creation of lye and higher proof alcohols, however, requires a bit more human intervention.
The bombardier beetle produces two chemicals that when combined, have a violent reaction. What material combinations (from anywhere in the animal and plant kingdom), without processing (other than harvesting it), could also give such reactions, and what might those theoretical limits be? Would these materials come from simpler organisms (yeast produces alcohol) or from more complex ones (bees creating honey)?
My initial guess is that creatures in the same environment wouldn't generate byproducts that would cause energetic reactions; you'd have to be clever and use animals or plants from different parts of the biosphere. But perhaps, there might be a combination of plants/animals that did things like create the equivalent of forest fires in a world devoid of lightning, perhaps.