The number of vendors providing storage solutions is huge and growing. There insn't one best solution, each vendor has advantages and disadvantages over the others, so the best solution for your company depends on many factors in your environment. There are even free open-source solutions for providing shared storage for backup or other non-production applications.
Types of shared storage: (protocols)
Storage Area Network (SAN) - Most common legacy solution. Uses Fibre-channel (FC) or iSCSI connections. FC still used where cost is not a factor in the highest performance environments. Requires special training and special fibre switches. iSCSI is becoming the new default choice and is much less expensive due to use of standard Ethernet hardware.
Network Area Storage (NAS) - New high-performance NAS units can compete with SAN storage. Typically use NFS connections over standard Ethernet. NOTE: not to be confused with inexpensive consumer NAS boxes which would not have the performance required for servers.
ATA over Ethernet (AOE) - Low level (non-routable, non-IP) storage over Ethernet that provides very low cost raw storage without management. Originally used in the scientific community to provide petaBytes of storage, but now becomming more mainstream. Many companies are adding management heads to make this solution comparable with other solutions above.
Virtualized Storage - Uses storage from many desperate physical units and makes them appear as one single block of storage.
Of particular interest to SMB's is VMware's Virtual Storage solution that provides a Virtual SAN from local disks in eah Hypervisor host. This is limited to only 2 or 3 host clusters, and not scalable beyond that.
Types of shared storage: (technology)
SAS/SATA disks - Common rotating media solutions provide up to ~200 IOPS per each SAS disk spindle. RAID-5/6 adds significant IO penalty, especially to writes. Optional cache memory may improve random IO performance.
Hierarchal Storage Management - Also called "Auto Tiered" These provide automatic movement of data between high-speed (high-cost) and lower-speed (lower cost) storage as required by actual usage.
Solid State Disk - These solutions are 100% solid state disk and provide 100X the IOPS of rotating media. The rapidly falling cost of these solutions is actually making them competitive for some workloads.
Open Storage - This is a software-only solution that runs on commodity hardware. The leading software in this category is Nexenta which then certifies dozens of hardware vendors to provide complete solutions.
Enterprise units are usually dual-head, and can fail-over from one controller head to the other on the fly without serious interruption of data. Single controller units are typically used for non-production or backup applications. One vendor uses a technique that could be termed Network RAID which combines (3) or more single-head units into a redundant cluster at the expense of having three heads and three sets of disks for each volume.