most (not all, some contributions are QTVR-only due to historical reasons)
Javascript + HTML5 / WebGL
many, but depends on photographer's choice of file upload (newer event = more likely to work) and your device's capabilities. You may experience slow playback, distorted images or unexpected browser quitting if there is only limited memory/GPU power available.
This was my first panorama of 2005 and a year later remains a favorite. Taken a few minutes after midnight in the midst of fireworks and confetti raining down on the crowds gathered in Times Square on New Year's Eve.