ARM is a major competitor in the world of processors for portable and embedded devices such as cell phones and e-book readers. ARM does not sell processors directly; rather, it licenses its designs to manufacturers such as Texas Instruments, which include the processors in Systems-on-Chip (SoC). SoCs comprise processors and peripherals such as power and I/O ports (similar to a CPU and motherboard on a traditional personal computer).These SoCs provide capabilities comparable to a desktop PC of several years ago, in a low-power, compact (deck of cards) package, and it is possible to run general-purpose operating systems such as Linux on them.
There are a number of development boards available to hobbyists interested in exploring embedded/SoC development (the author has no affiliation with any of the companies listed herein):
BeagleBoard (Cortex-A8/OMAP3-based, community-based).
PandaBoard (Cortex-A9/OMAP4-based)
Industrial ARMWorks (Samsung S3C6410A)
Hammer (Samsung S3C2410A, DIP 40)
Further reading on ARMdevices trade blog.