EHCache
EHCache is a pure Java, in-process cache with the following features: Fast,Simple, Acts as a pluggable cache for Hibernate 2.1., with Small foot print, Minimal dependencies, Fully documented and Production tested.

Java Caching System
JCS is a distributed caching system written in java for server-side java applications. It is intended to speed up dynamic web applications by providing a means to manage cached data of various dynamic natures. Like any caching system, the JCS is most useful for high read, low put applications.

JCache
JCache Open Source is an effort to make an Open Source version of JSR-107 JCache. Since the JSR-107 hasn&t released any specs for years, This version still builds on the original Functional Specification.

OSCache
OSCache is a widely used, high performance J2EE caching framework. In addition to it's servlet-specific features, OSCache can be used as a generic caching solution for any Java application.

ShiftOne
ShiftOne Java Object Cache is a Java library that implements several strict object caching policies, as well as a light framework for configuring cache behavior.

SwarmCache
SwarmCache is a simple but effective distributed cache. It uses IP multicast to efficiently communicate with any number of hosts on a LAN. It is specifically designed for use by clustered, database-driven web applications.

TreeCache / JBossCache
JBoss Cache is a replicated and transactional cache that allows you to cache data from Enterprise Applications in order to dramatically improve performance. The caching data is replicated automatically allowing you to easily cluster data across a grid of JBoss servers. JBoss Cache can run as an MBean service within JBoss Application Server or other J2EE containers. It can also run standalone.

WhirlyCache
Whirlycache is a fast, configurable in-memory object cache for Java. It can be used, for example, to speed up a website or an application by caching objects that would otherwise have to be created by querying a database or by another expensive procedure. From the testing that we have done, it appears to be faster than any other Java cache that we have been able to inspect.