Emulator Summary

Z80 Spectrum

Zesarux - a good Windows (and Linux) Z80 emulator with a wealth of command-line options. Good for testing Z80 Assembler.

Zero Emulator - a good Z80 Spectrum emulator, written in C#, that runs on Windows.

x86

Qemu -  a good emulator that runs on Linux. Good for x86 assembly.