While it's quite common now for modern video game consoles to come pre-built with the ability to play movies, back in the days of the Atari 2600, the hardware's limitations meant that people never even dreamed about the possibility of being able to watch films on the device.
Yet, despite that, a creator named Lodef Mode has recently developed a cartridge named MovieCart that allows the old machine to do just that (as spotted by techeblog & Hackaday!).
As Hackaday notes, the quality of the video and sound quality isn't necessarily the best, with the image, in particular, being subject to the colour limitations of the console and a resolution of just 80×192. But it's still a fun project nonetheless and one we imagine will interest Atari fans who likely never considered this a possibility.
In order to get the videos to work, you will need to encode your own video file to the correct format (an encoding tutorial is available here) on a Mac or PC and then drag that file over to a MicroSD card that you can plug directly into the MovieCart. You should then be able to play the movie on the Atari 2600 (ColecoVision Atari Adapter or modded Flashback 2).
MovieCart packages, including a fully assembled PCB board, an SD card featuring "Night of the Living Dead", and cart labels, were available to pre-order on Tindie for $24.99. However, they appear to be sold out at the moment. There is a waitlist option, however, should you want to be alerted when they're back on sale.