Spring Expression Language (SpEL)
Expression Language API (2.1 Maintenance Release)
MVEL is a powerful expression language for Java-based applications. It provides a plethora of features and is suited for everything from the smallest property binding and extraction, to full blown scripts.
Expression Language 2.2 Implementation
OGNL - Object Graph Navigation Library
Expression Language Implementation
The Commons Jexl library is an implementation of the JSTL Expression Language with extensions.