Forget the MP3 player, or even the Walkman -- the real instigator of the portable audio revolution is the compact cassette, which just marked its 50th birthday. Philips formally launched the format on September 13th, 1963, bringing recorded sound to a truly portable (and more accessible) form factor. The technology didn't just kickstart the markets for media players, field recorders and boomboxes; it led to bootlegs, mixtapes and other ways to shake up the audio status quo. The cassette has largely disappeared outside of nostalgic reissues and transforming robot toys, but its effects are still visible after half a century of progress.
[Image credit: Tony Unruh, Flickr]