A guitar string oscillating at it's lowest note.
Any system that is free to move (think washing line) and has some mechanical energy input (e.g. wind) will rock to and fro. Tall buildings, bridges and wind turbines are all designed to withstand this.
You can get more complex waves (more accurately half waves) in these systems if you restrain the oscillator half way along e.g. above the 12th fret on a guitar - you have produced a harmonic that will have twice the frequency (I.e. one octave higher pitch.)