Primitive-type collections based on CERN's Colt Java API
Persistent collections based on 2-3 finger trees.
GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework.
Eclipse Collections API
OPS4J Base - Utilities/Extensions related to java collections.
Efficient collections for sets and maps that has primitive types as keys.
Jcommon Collections Package
Commonly used utility classes (java.util.Collections related)
An experimental foundation library for Scala focussed on efficiency and clean design.