IPod

An iPod was a music storage and listening device from 21st century Earth.

Owen Harper listened to "Atlas" by Battles on his iPod while cleaning his flat. (TV: A Day in the Death)

After the death of Danny Pink, Clara Oswald observed that people "just kept walking with their iPods and their shopping bags." (TV: Dark Water)

In 5,000,000,000, Cassandra presented to fellow guests on Platform One what she identified as an iPod. With it she played Soft Cell's "Tainted Love", a traditional song, and Britney Spears' "Toxic", a ballad from Earth's classical era. (TV: The End of the World)

Other similarly named devices from that era included the iPad and the iPhone. (TV: Dark Water, Kill the Moon)

Behind the scenes
Though not made clear, iPods are an example of MP3 players. The "iPod" seen in The End of the World is in fact a jukebox, which was invented several decades earlier. This was meant as an anachronism, since Cassandra is boldly making claims about forgotten Earth history. To contemporary viewers, this would have been more than obvious, so this is not expanded upon. Technically, in the Doctor Who universe, this may well be an iPod.

IPod