There are numerous players in the storage virtualization market today, but I can categorize them as in-band vendors (typically appliances), out-of-band vendors and Split Path Architecture for Intelligent Devices (SPAID) vendors.
StoreAge Ltd., which was recently acquired by LSI Corp., has essentially been the only out-of-band product in the industry. The Storage Virtualization Manager (SVM) product runs on its own Intel server-based appliance. It has also recently ported that software to purpose-built appliances from QLogic Corp. and Emulex Corp.
The SPAID category of products includes products like Network Storage Platform (NSP) from Incipient Inc. and Invista from EMC. These are similar to an out-of-band architecture, but both are designed to utilize an intelligent switch in the network environment. For example, both Incipient NSP and EMC Invista work with Cisco Systems Inc./Brocade Communications Systems Inc. intelligent switches.
Hitachi Data Systems (HDS) represents a small fourth category of storage virtualization where the feature is actually implemented within the storage cabinet itself. The TagmaStore system is one good example of this approach.
Check out the entire Storage Virtualization FAQ guide.