The main goal of the Linux Trustees project is to create an advanced permission management system for Linux. The solution proposed is mainly inspired by the approach taken by Novell Netware and the Java security API. Special objects (called trustees) can be bound to every file or directory. The trustee object can be used to ensure that access to a file, directory, or directory with subdirectories is granted (or denied) to a certain user or group (or all except user or group). Trustees are like POSIX ACLs, but trustee objects can affect entire subdirectory trees, while ACLs a single file. Trustees works with the 2.6 Linux kernel.
The goal of the dtfs project is to implement a log-structured file system within the Linux 2.2.x kernels. dtfs has a filesystem-independent core that provides general services required for a log-structured file system and uses a "traditional" file system implementation to do the actual filesystem/VFS operations. The tradtional file system of choice is currently Linux' ext2 file system. Using the ext2 file system together with the log-structured core should both reduce the implementation work required to be done and facilitate the future maintainence of dtfs.
The Extent File System (efs) is Silicon Graphics' early block-device filesystem, widely used on pre-6.0 versions of IRIX. Since 6.0, xfs has been bundled with IRIX and users are being encouraged to migrate to xfs filesystems. IRIX support for efs will be read-only in versions of IRIX beyond 6.5, however efs is still very much in use on SGI software distribution CDs.
Rio (RAM I/O) for FreeBSD is an implementation of the Rio file cache project at Michigan. The basic idea of Rio is to make memory as safe as disk from operating system crashes. Such "reliable main memory" is useful in a variety of contexts, e.g. file systems, transaction systems, or checkpointing.