("Journalling Flash File System version 2 or JFFS2 is a log-structured file system for use with flash memory devices.")
It worked fine on my One Laptop Per Child laptop for years until it didn't, and there's no utility to repair it; neither Wikipedia nor its FAQ mention this absence. Fortunately (?) userspace has no idea of the carnage going on below it, so I could tar off my files despite all the "jffs2_get_inode_nodes: Eep. No valid nodes for ino #340448" syslog messages.