typelevel/scalacheck

Property-based testing for Scala

milessabin/shapeless

Generic programming for Scala

scopt/scopt

command line options parsing for Scala

lihaoyi/fastparse

Writing Fast Parsers Fast in Scala

fthomas/refined

Simple refinement types for Scala

typelevel/simulacrum

First class syntax support for type classes in Scala

lihaoyi/utest

A simple testing framework for Scala

scala/scala-parser-combinators

simple combinator-based parsing for Scala. formerly part of the Scala standard library, now a separate community-maintained module

lihaoyi/scalatags

ScalaTags is a small XML/HTML construction library for Scala.

typelevel/discipline

Flexible law checking for Scala

typelevel/squants

The Scala API for Quantities, Units of Measure and Dimensional Analysis

outr/scribe

Practical logging framework that doesn't require any other logging framework and can be completely configured programmatically.

typelevel/machinist

Spire's macros for zero-cost operator enrichment

lihaoyi/fansi

Scala/Scala.js library for manipulating Fancy Ansi colored strings

monix/minitest

The super light testing library for Scala and Scala.js

suzaku-io/boopickle

Binary serialization library for efficient network communication

alexarchambault/case-app

Type-level & seamless command-line argument parsing for Scala

scala/scala-collection-compat

Scala 2.13 Collection Compatibility Library

propensive/contextual

Contextual

scodec/scodec-bits

Provides immutable datatypes for working with bits and bytes