Plugins to Learn

jQuery Validate

http://static.springsource.org/pitchfork/ - for Spring / JEE integration

uncss

Specifications to Learn

EJB 2.x

X/Open XA

APIs to Learn

JMS

ASP.NET Web API 

Technologies to learn

Apache Kafka

http://www.autohotkey.com/

OData

Moose 

implementations for JPA are: • Hibernate • OpenJPA • Toplink Essentials • ObjectDB

JMX mbeans to do hardware device control

MSMQ & JMS

Maven

Apache Camel

BPMN (standard based) - Drools, Activiti, JBPM, Bonita

JPA 

Coherence (distributed caching)

JCache 

 Java Expert System Shell (JESS)

EhCache (distributed memory cache)