Plant biostimulants offer promising opportunities to aid in addressing the challenges faced by contemporary agriculture – maintaining viable yields of high-quality produce while reducing the environmental footprint of production. Biostimulants have been shown to enhance plant growth, boost yields, and increase crop tolerance to abiotic stress, acting via a number of mechanisms including phytostimulation, biofertilization, and biocontrol. In the European Union, frameworks and industry standards that require rigorous scientific evidence for all claims have been introduced as a requirement for product registration. However, a lack of regulation in many countries, including Australia, has led to significant variation in product quality and inconsistent results in the field, issues that remain to be addressed. As industry develops the next generation of biostimulant products, the potential for synergistic effects when applying different categories of biostimulants in combination has generated particular interest. In the realm of microbial inoculants, the formulation of microbial consortia which look to harness the beneficial action of multiple species is being investigated. The potential for biostimulants to add value to soilless production systems is another area for development in this space. As biostimulants encompass a diverse group of products, this review focuses on three categories: microbial inoculants, humic substances, and seaweed extracts. After summarising the current understanding of how these substances operate, as well as the crop responses they have been shown to elicit, we examine the challenges facing the development of more effective and reliable products and explore some of the key areas for future research.