There are a few products the use primarily DRAM, usually backed by capacitors/batteries and flash.

ZeusRAM SAS drive: http://www.stec-inc.com/product/zeusram.php

RamSan rack-mounted: http://www.ramsan.com/products/rackmount-ram-storage/ramsan-...

Kaminario K2 SAN: http://www.kaminario.com/products/K2-Solid-State-SAN-Storage...

acid-state, http://acid-state.seize.it/, which was previously known as happstack-state, and before that HAppS-State, has been doing it since 2005 or so.

acid-state currently lacks replication/multimaster support, but happstack-state has had several experimental implementations of that as well.

acid-state is Haskell specific.. but that is part of the appeal. You can directly store fancy algebraic data structures with acid-state. You are not limited to a simple combination of records integers and strings (for example).

SATA2 DDR2 HyperDrive5 64GB: A Solid State DDR drive.

http://www.hyperossystems.co.uk/07042003/hardware.htm

Something like http://www.anandtech.com/show/1742 seems similar.