Commons Collections w/ Generics
by Tim O'Brien
Related link: http://sourceforge.net/projects/collections
Commons Collections has a ubiquitous presence in Java programming, and I haven't seen a single project in the last four years that hasn't included this library. For those of you who are looking for a version of Commons Collections with support for Java 5 generics, take a look at Commons-Collections with Generics.
Jaggregate as an alternative collections API
I would also encourage any interested parties to have a look at Jaggregate (http://jaggregate.sf.net), a Java-5-only collections library modeled after the Smalltalk collection protocols.