Several utilities for and around the Java Collection API
This library provides efficient implementations for various collection data structures (esp. linked lists and priority queues), which have very insufficient implementation in the Java Collections Foundations that makes it hard or even impossible to exploit their efficiencies.
Jcommon Collections Foundation Package
Utility library for collections, comparators and iterator classes
Collections utilities which build on top of Guava and the standard collections library.
Collections library of the Staccato-Commons project, focused on providing new abstractions that mix object oriented and functional programming style for dealing with iterable objects.
Brownies Collections complements the Java Collections Framework. GapList combines the strengths of both ArrayList and LinkedList. BigList is a list optimized for storing large number of elements. There are specialized List implementations for all primitive data types. The key collection classes offer support for keys and constraints for lists and collections.
Java Collections which are designed to be huge (may GB)
Persistent (immutable) collection library for Java
Carefully designed and efficient extension of the Java Collections Framework with primitive specializations and more, built for Java 8 (API)