HPFS, the High Performance File System, was introduced in OS/2 as an alternative to FAT. It's design is quite clean, and has an eye on performance. There is also support for extended attributes, apparently used by OS/2's graphical shell, the Workspace Manager.

HPFS preserves the DOS boot sector in sector 0 and puts its own superblocks into sectors 16 and 17 (that is, 8K into the device). The space in between is used for boot code. All structures in HPFS are clearly marked with magic numbers.

The JFS file system was originally created for AIX, IBM's flavor of Unix. It was later made more portable and was released with OS/2 Warp. IBM also ported it to Linux and released it under the GPL. The main feature of JFS is meta-data journaling.

JFS puts its superblock 32K into the device. It uses extents, inodes, and B+ trees.

Information on JFS (at least the open-sourced Linux implementation) can be found at http://oss.software.ibm.com/jfs/.