OSCache is a Java framework developed by OpenSymphony that makes it easy to cache content in Web applications. The project was retired in May 2011 when OpenSymphony announced  its closure. While some OpenSymphony projects continue elsewhere having been picked up by external parties, OSCache has not been so fortunate. The source code is, however, available via the OpenSymphony website.
According to OpenSymphony, OSCache
is a caching solution that includes a JSP tag library and set of classes to perform fine grained dynamic caching of JSP content, servlet responses or arbitrary objects.
provides both in memory and persistent on disk caches.
can allow your site to have graceful error tolerance (e.g. if an error occurs like your db goes down, you can serve the cached content so people can still surf the site almost without knowing).