Sinatra
Classy web-development dressed in a DSL
...Its DSL maps routes directly to Ruby blocks, making it straightforward to build APIs, small services, and quick prototypes without the overhead of full-stack frameworks like Rails. Despite its simplicity, Sinatra supports middleware, templates, sessions, filters, and helpers, so it can grow with more complex requirements. The framework is Rack-based, which means it is highly compatible with the Ruby web ecosystem and can plug into larger applications as a lightweight component. Because of its low footprint, it’s ideal for microservices, command-line dashboards, and educational projects.