Anaerobic digestion is a promising option for recycling agricultural by-products and some organic wastes. While both agricultural by-products and wastes have no direct commercial value, their management is both complicated and costly. One option to simplify by-product management and reduce the costs associated with biogas plant feedstock is to substitute dedicated crops with vegetal by-products. Given that the chemical composition of some of these by-products can differ considerably from more typical biogas plant feedstock (such as maize silage), more complete knowledge of these alternatives to produce environmentally friendly energy is warranted. To this end, batch trials under mesophilic conditions were conducted to evaluate the potential biogas yield of many agricultural by-products: maize stalks, rice chaff, wheat straw, kiwi fruit, onions, and two expired organic waste products (dairy and dry bread) from the retail mass-market. Among the considered biomasses, the highest methane producer was the expired dairy product mixture, which yielded 554 lNCH4 kg−1 volatile solids (VS). Maize stalks and wheat straw produced the lowest yields of 214 and 285 lNCH4 kg−1VS, respectively. An assessment of the biogas and methane yields of each biomass was also undertaken to account for the specific chemical composition of each biomass as it can affect the anaerobic digestion operating system. Finally, the total Italian green energy production that might be derived from feeding all these biomasses to a biogas digester was estimated, in order to understand its potential impact.