Spring

In short, Spring is a lightweight[1] J2EE framework that uses AOP quite a lot. It can be used with or without EJBs.

Some further insights: Spring is a collection of APIs and a framework. The core is the bean factory, which uses XML-defined mappings of IDs to class (or class factory) names to manage beans. Patterns like singleton, dependency checking etc. can be defined in XML. Property and constructor values can be set on these beans, including references to other beans. Thereby, whole graphs of beans can be created.

Homepage: http://www.springframework.org/

[#1] Well, as light as frameworks get these days!