Google App Engine Virtual File System (GaeVFS)

http://code.google.com/p/gaevfs/

GaeVFS is an Apache Commons VFS plug-in that implements a distributed, writeable virtual file system for Google App Engine (GAE) for Java. GaeVFS is implemented using the GAE datastore and memcache APIs. The primary goal of GaeVFS is to provide a portability layer that allows you to write application code to access the file system--both reads and writes--that runs unmodified in either GAE or non-GAE servlet environments.

Start with the Using GaeVFS Wiki page to learn more about GaeVFS.