Fly.io is a platform for deploying and running containerized applications, offering features such as creating a Docker image, pushing it to the registry, and automatically launching the web application. It's gaining popularity among developers due to its fast, reliable, and secure nature. Fly.io provides competitive pricing, making it an attractive option for developers looking for a free or low-cost platform. To deploy FusionAuth on Fly.io, users need to create a Postgres database, set up a proxy connection to the local instance, and allocate an external IP address if needed. The application is then launched using the Dockerfile and redeployed as necessary. Users can scale the instance by running specific commands and access their application via the provided URL.