TL, DR: world pervasive algorithms (FFT-related)!
The continuous Fourier transform, the Discrete-time Fourier transform (DTFT) and the Discrete Fourier transform (DFT) share conceptually similar traits (regarding energy, convolution, shift, scale, etc.)
The DFT unveiled a very scalable and fast algorithm to put those concepts into practice: the FFT. It is considered by many to be one of the most important algorithms, and is in the lists below:
Of course, Google PageRank, the Euclidian division, Quicksort, Kalman filtering are used daily. But if we include digital data compression, for instance: MP3 Compression/The 10 Algorithms That Dominate Our World, or JPEG decoding, as while not directly DFTs, but very close avatars which might not have been discovered without the DFT, I am pretty sure that billions of DFT-inspired algorithms are run every hour.
In 2012 (source: A Faster Fast Fourier Transform: New algorithm crunches sparse data with speed), Richard Baraniuk was already credited with "FFTs are run billions of times a day", and Gilbert Strang:
once referred to the fast Fourier transform, or FFT, as “the most
important numerical algorithm in our lifetime"