IBM Mirage. Replaces standard formats with their own image format (“MIF”), which is a tar-like format. Advantages are you can easily see into and modify the disk image. Also supports sharing common data between VMs. Disadvantage is that it’s entirely unlike the disk image formats that are already out there, so it seems unlikely to gain traction. Also claim to be doing package upgrades on offline guests which is something libguestfs users have been doing for well over a year.
The paper above mentions the following related work:
Ventana (Stanford). Paper is undated, but latest reference is to 2006. A virtualization-aware filesystem (not block device) which offers lightweight snapshots and branches. Concept seems to be similar to wandering-tree filesystems (most notable modern example being btrfs, but these have been around for a long time).
Machine Bank (Microsoft). Microsoft’s VHD (virtual disk format) with support for sending these around a network, partial caching and so on.
Moka5 engine. Some sort of VDI thing. No information on the website.