MogileFS

MogileFS is written in Perl and has been around for awhile:

http://www.danga.com/mogilefs/

http://www.capoune.net/mogilefs/

WebDAV

http://php-webdav.pureftpd.org/project/php-webdav

MySQL FileSystem?

http://www.onlamp.com/pub/a/php/2000/09/15/php_mysql.html

This looks great:

http://forge.mysql.com/wiki/BLOB_Streaming_For_PHP

http://www.blobstreaming.org/index.php

Memcache FS?

http://memcachedb.org/