well... that's a very broad question, but the generally accepted idea of an emulator usually pertains to gaming. You can get emulators for all of the old consoles, and most if not all of the original games that were played on them, from the Atari 2600, right up through DreamCast and beyond. I have emulators and games for Commodore64, Nintendo, and Sega Genesis.
They are generally free, but you have to look around a little. Check out some of the popular p2p sharing programs out there. Of course, in order for you to know exactly what you're looking for, is your best friend. Never fear it, for it shall lead the way to your very dreams!
Also, bear in mind, that obtaining and using the games to play on these emulators is considered illegal in the same fashion that downloading mp3s without paying for them is illegal, except that there is no way that I know of to legally buy a gamefile for an emulator.
/specializes in drama when required