The ARM family of processors are widely used by Single Board Computer manufacturers. ARM processors are extremely popular for embedded systems development and are common in devices such as cell phones and portable instruments due to their low power capabilities.
There are many ARM processor manufacturers, called "licensees" since ARM licenses out the rights to create processors based on the ARM cores. This strategy allows silicon companies to take advantage of the power and broad range of functions in the ARM processor line and to focus all their efforts on creating added value (on-chip functions and I/O) for their variant of the chip.
There are many development tools and software environments that support the ARM core which also makes it an excellent choice.