OffsetFS allows you to mount an arbitrary file or blockdevice as a
directory tree of read-only files that each represents the mounted file
starting at different offset.
BDwrap is a template for writing stackable blockdevice drivers on Linux. It's clean interface hides the complexity of device handling, asynchronous calls, buffer cache interaction, etc. It is a great tool to develop stacked block drivers quickly.
The Vinum Volume Manager is a device driver which implements virtual
disk drives. It isolates disk hardware from the blockdevice
interface and maps data in ways which result in an increase in
flexibility, performance and reliability.