ARM - Acorn Risc Machine

ARM is a RISC processor with the following features
Load-store architecture
16 registers, each 32-bits wide
Three-operand instructions, 32-bits wide
At each clock cycle a new instruction is issued
Low energy consumption (W/MIPS)

It is designed by ARM Ltd. and licensed to manufacturers
ARM7: nommu, very widespread as a microcontroller
DEC, then Intel: StrongARM (now dead)
Intel, then Marvell: Xscale: 80200, PXA255, PXA270, IXP425, ...
Cirrus Logic: EP93xx: 9302, 9315, ...
Freescale (now NXP): iMX1, iMX21, iMX27, iMX31, ..., iMX7, ...
Atmel: AT91SAM: many flavours
... many more ...

http://en.wikipedia.org/wiki/ARM_architecture