AspectJ
AspectJ is a seamless aspect-oriented extension to the Java programming language, Java platform compatible and easy to learn and use. AspectJ enables the clean modularization of crosscutting concerns such as: error checking and handling, synchronization, context-sensitive behavior, performance optimizations, monitoring and logging, debugging support, multi-object protocols.

AspectWerkz
AspectWerkz is a dynamic, lightweight and high-performant AOP framework for Java. AspectWerkz offers both power and simplicity and will help you to easily integrate AOP in both new and existing projects. AspectWerkz utilizes runtime bytecode modification to weave your classes at runtime. It hooks in and weaves classes loaded by any class loader except the bootstrap class loader. It has a rich and highly orthogonal join point model.

CAESAR
CAESAR is a new aspect-oriented programming language compatible to Java, that is, all Java programs will run with CAESAR.

Colt
Open Source Libraries for High Performance Scientific and Technical Computing in Java.

DynamicAspects
DynamicAspects enables you to do aspect-oriented programming in pure Java. Using the "instrumentation" and "agent" features introduced with Sun JDK 1.5, aspects can be installed and deinstalled during runtime!

Dynaop
Dynaop, a proxy-based Aspect-Oriented Programming (AOP) framework, enhances Object-Oriented (OO) design in the following areas: code reuse decomposition dependency reduction.

EAOP
Event-based Aspect-Oriented Programming (EAOP) for Java.

JAC
JAC (Java Aspect Components) is a project consisting in developing an aspect-oriented middleware layer.

JBossAOP
JBoss-AOP allows you to apply interceptor technology and patterns to plain Java classes and Dynamic Proxies.

Nanning Aspects
Nanning Aspects is a simple yet scaleable aspect-oriented framework for Java.