Nestopia is a Nintendo Entertainment System (NES) emulator for the Mac. It allows you to play ROM files, which are dumps of NES games. You are legally allowed to play any ROM files of games that you own. The emulator doesn't require any configuration at all. You can just load a ROM and start playing.
NES emulators have been around for a long time, and they have got the emulation of the NES down to an art. NES games are played greatly. Of course there is sound, customizable controls and support for joysticks. You can also freeze the gamestate to continue later. When you do this, a new file is saved to your computer.
I particularly like the way you can map the keyboard controls by dragging and dropping. This is something I had not seen before and it is a very easy way to do it. The NES only had like 4 buttons, and I found that using the "A" and "S" keyboard buttons worked best for me.
There are a bunch of settings that you can change from the Preferences window. You can, for example, change the speed of emulation, enable or disable sound and change the volume, and change some video options (frame rate, effect size, type, resolution, etc).
In short, this is a perfect NES emulator. It is free. It works like a charm and it should open any NES rom.