Ataroid was a big hit when it first came out. It was not perfect and it is far from perfect today too, but it did a pretty good job back then. The emulator is based on the traditional Stella code. It runs as an Atari 2600 emulator and brings in access to a bunch of games and applications. The emulator was released for Android. While there are more sophisticated options out there, it is still a classic that will bring back some good memories.
The emulator is free of charge. Anyone can download it on their device. The download file is relatively small – less than an MB – and widely available online. The installation should take less than a minute. Simply unzip the emulator and run it straight away. You are more likely to find the 1.0.4 version as an APK file, which is the most complete one out there.