In my experience, the following libraries and tools help you build Scala applications very quickly. They all require miminal setup ceremony and target the most common use-cases, without the user needing to learn new domain specific languages. Many of them take inspiration from Python's philosophy of keeping things simple (yet they are still statically typed). As such, they may not be all you ever need, but they should help you build solid applications relatively quickly.
These are of course subjective recommendations, including shameless plugs.
- Build tool: mill
- Unit testing: utest
- Configuration and command line parsing: cmdr
- JSON: upickle
- File IO and system: os-lib
- Relational database access: quill (for Scala 2) / simplesql (for Scala 3)
- Web server: cask
- Web client: requests-scala
- Actors framework: castor
- Protocol buffers and gRPC: ScalaPB
- Metrics collection: ustats