Or an active-active fail-over cluster à la VMware.
Today I have learned about a totally crazy/cool looking architecture where the expensive VMware ESX server was replace by a free/cheap VMware Workstation version in combination with DRBD.
Basically DRBD we name the poor man's SAN and that is exactly what this customer is doing. He replaced the SAN with DRBD and now he can easily move one VMware instance to the other host. Possibly it is not that flexible and powerful as an ESX Server but also not so expensive...
The architecture looks as follows: