The standard Scala XML library.
As of Scala 2.11, this library is a separate jar that can be omitted from Scala projects that do not use XML. We're also looking forward to alternative implementations!
To depend on scala-xml in SBT, add something like this to your build.sbt:
libraryDependencies += "org.scala-lang.modules" %% "scala-xml" % "1.0.2"
(Assuming you're using a scalaVersion for which a scala-xml is published.
The first 2.11 milestone for which this is true is 2.11.0-M4.)
To support multiple Scala versions, see the example in https://github.com/scala/scala-module-dependency-sample.