most (not all, some contributions are QTVR-only due to historical reasons)
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.
Date/Time: September 28, 2011, 10:30 UTC (12:30 local time)
Equipment: Taken with a Nikon D300 + Nikkor 10.5mm DX AF Fisheye mounted on a Nodal Ninja 3 MKI head.. Two rows of six shots +/-30 degrees. Optimized with Photoshop/Camera Raw, stitched with PTGui and KRPano Tools.