Karplus–Strong
A synthesis algorithm that sounds like a plucked string by circulating a short burst of noise through a delay line with a simple lowpass filter in the feedback path.
Kevin Karplus and Alexander Strong published it in Computer Music Journal in 1983. Fill a delay line with random values, then for each new sample feed back the average of two adjacent ones. Averaging is a lowpass filter, so the treble decays faster than the bass, just as on a real string. The length of the line sets the pitch: at 44.1 kHz, about 100 samples gives you 440 Hz.
It costs a couple of additions per sample, which made it runnable on hardware that could manage little else. Integer delay lengths go audibly out of tune in the upper register, a problem David Jaffe and Julius O. Smith fixed with allpass filters the same year. The algorithm became the gateway to physical modelling and remains a classic first exercise in audio programming.