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Fusion-io pioneered this form factor for servers. Their design uses ECC for all DRAM buffers - a nice detail if data integrity is vital - but you pay for the quality. Most system vendors OEM Fusion-io for their servers.
Virident - who won a big investment from Seagate - has PCIe SSDs in a variety of capacities. Usually less expensive than Fusion-io.
If you are running a storage area network - SAN - you might want a shared resource accessible over Fibre Channel. You'll pay enterprise prices, but the performance is stunning.
Violin Memory. Their unique architecture doesn't use SSDs. Instead they've designed a system that goes direct to the flash chips and offers excellent and reliably low latency.