Embedded Handbook/Emulators

Software emulation of systems can often times be as good (if not better) than the real thing.

Qemu

A generic and open source machine emulator and virtualizer for x86, x86_64, arm, sparc, powerpc, mips, m68k (coldfire), and superh.

Armulator

Emulate armnommu/uClinux (no-mmu Linux) in GDB.

Hercules

Hercules System/370, ESA/390 and zArchitecture Mainframe Emulator.