We have developed an online oscilloscope program which allows users to see waveforms by utilizing their computer microphones, selecting from our library of over 30 audio files, and opening any *.mp3 or *.wav file on their computers. The oscilloscope displays real-time signals against time. The oscilloscope has been calibrated so one can make accurate frequency measurements of periodic waves to within 1%. The web app is ideal for computer projection in class.