MESS - Multi Emulator Super System
Wether your childhood home computer was a BBC Micro, Commodore 64/128, Dragon 64, Amiga, Atari, ZX Spectrum, ZX81 or your console of choice was SNES, Gameboy, MegaDrive or Epoch Game Pocket Computer (what do you mean never heard of it!?), MESS almost certainly has it emulated.
Actually it's probably not worth listing all the systems that Multi Emulator Super System supports. Over 550 unique systems, and over 1460 variations and is growing all the time.
Although it should be pointed out that not all of the systems in MESS are functional, some are marked as non-working or are in development.
The project strives for accuracy and portability and therefore is not always the fastest emulator for any one particular system, but if you want an emulator that covers everything then you should look no further.
From the Wiki: "MESS is a source-available project which documents the hardware for a wide variety of (mostly vintage) computers, video game consoles, and calculators through software emulation, as MAME does for arcade games. As a nice side effect to this documentation, MESS allows software and games for these hardware platforms to be run on modern PCs. "
Sources for Commodore 64 emulator games.