In the 1950s, one of the biggest uses of synthesizers in mainstream culture was the application of theremins in to mainstream culture.  Many movies such The Day Earth Stood Still (1951) and The Ten Commandments (1956) utilized the unique and eery sound the theremin produced.
 

The 1960s brought many things, but in the world of synthesizers, two big contributions included the Bradmatic Mellotron and the Moog Modular System.  The Mellotron was an quirky keyboard that imitated multiple different sounds by using prerecorded tape loops including sounds like trumpets, strings, and woodwinds.  The Beatles extensively used the mellotron in songs such as: "Strawberry Fields Forever," "Yellow Submarine," and "Lucy in the Sky With Diamonds."  The Fab Four were attracted to this synth because of the rich, warm tones that this famous synth produced.

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The other large contribution to synth technology was the Moog Modular System.  It used multiple different cables to patch multiple different oscillators, filter banks, and voltage controlled oscillators to make one of the most recognizable tones ever used in the industry.  One of the most well known songs of the 60s to use a Moog Modular was "Strange Days" by The Doors.  The song used this to make psychedelic sounds that were totally different for the time.  Because of its simple design and use of patch cables, the Moog Modular System was easy for the user to customize to make creative new tones. 

 

Patch cables are kind of like a mid 20th century telephone operating console where a user connects different components of the system together.  In the Moog, the Oscillators, filters, sequencers, keyboard, and voltage controlled oscillators are not already patched, so in order for the player to make noise, the system "programmed" with these patches.