PromptBase is an online marketplace where users can buy and sell AI prompts for various platforms like Midjourney, ChatGPT, DALL·E, and Stable Diffusion. It offers over 150,000 curated prompts created by expert AI developers. Users can explore different categories such as art, logos, marketing, productivity, and more. PromptBase also allows users to create AI apps and hire prompt engineers for custom projects. It's a valuable resource for creators looking to enhance their work with ready-to-use prompts.