VR Panorama Viewers
Quicktime is the oldest panorama viewer, and it is installed on 100% of all Macintoshes, and approximately 60% of all PC's. It's slower than most of the newer viewers such as Immervision's PurePlayer, or PangeaVR, however, it is the highest quality viewer, and definitely the most stable. On reasonably fast computes, Quicktime can render panoramas full-screen at an acceptable frame rate.
Java is installed on 100% of all Macintoshes, and 99% of all PC's, so these viewers are the best to use when compatibility is your main priority. Unfortunately, Java has a fairly low memory limit, so high-resolution panoramas have to be reduced in order for Java to be able to display them. The most common Java viewer is PTViewer since it is free and has been around for a while, however, the performance is not very good, so it's only useful for fairly small panorama windows. The new PurePlayer viewer from Immervision is also free, but its performance is astounding, and panoramas can be displayed full-screen. The PurePlayer viewer also uses a compression technique to keep panoramas in the limited Java memory, but maintain more resolution than other viewers. You still don't want to go much larger than a 800x600 window with PurePlayer since the degraded resolution of the panorama will start to show up at that point.
PangeaVR is by far the fastest panorama viewer. It uses 3D hardware acceleration on the user's video card to draw the panorama, so the motion is liquid-smooth! Unfortunately, PangeaVR has to be installed by the user, so for visitors to see PangeaVR panoramas they have to install the PangeaVR plug-in. PangeaVR does not have a Java memory limit, but it does have a VRAM memory limit. The more VRAM the user has on their video card, the higher resolution the panorama will appear. Most panoramas will appear as good as Quicktime if there is 128MB of VRAM or more available. PangeaVR automatically reduces the resolution of the panorama based on the amount of available VRAM. The user experience with PangeaVR, however, is by far the best of all the viewers since the smooth interaction makes the panorama seem much more realistic. The PangeaVR plug-in can be downloaded here.