Nestopia is a portable emulator for the Famicom and NES, the Nintendo Entertainment System. It is an open-source project able to simulate the game consoles that were so popular back in the 80's and early 90's, enabling you to play classic titles like Super Mario, Contra, Battle City, Legend of Zelda... you name it. It offers an array of advanced options to make the experience as smooth and close (or even better) to the real thing as possible.
To play games in Nestopia you will need a ROM, that is a file containing the game itself. Nestopia supports ROMs with the formats NES, UNF, UNIF, NFS, FDS, etc. You will find thousands of free ROMs on the web, which usually come in RAR, ZIP, and 7z files that you don't need to decompress. Downloading ROMs is far from being legal, as you may guess; you're supposed to own the cartridge of the game you want to play.
As far as video goes, the emulator allows you to select the video card, choose from different video resolutions, and make adjustments like brightness, contrast, etc. It also supports free scaling letting you resize the window as you like. The program comes with several video filters too that you can use to smooth pixelation and enhance image quality. It offers excellent sound emulation too, with a few options to adjust.
From the 'Options' menu you can also configure the controls for the different game actions/directions to play with the keyboard, a gamepad, or both. A very interesting option that I hadn't found in emulators like VirtuaNES is the rewind feature. It basically allows you to reverse gameplay just by pressing the backspace key.
Additional options include the ability to save and load states, take screenshots, and record sound and video. There is also a dedicated section to save and edit cheat codes. Another feature to highlight is that the program supports netplay using the Kaillera network. For that you need to download the server app, as the client is already included in the pack.
All in all, if you're feeling a bit nostalgic for the old-school games, Nestopia is one of the best options to replay those classic games that never seem to get old.