FSPViewer is a free viewer for spherical (equirectangular) panoramic images. It is designed for viewing local (hard-disk, network or CD) high resolution panoramic images at full screen and with very high image quality. It uses advanced interpolation algorithms to show you smooth images without losing sharpness.

FSPViewer has been available for many years as a Windows-only program. Version 2 has been entirely rewritten and now the program is available for many operating systems.
I can also embed the rendering engine in other programs to add panoramic image viewing to custom software. Feel free to contact me if you are interested.

FSPViewer can handle images of any size: in a 64 bit computer it is limited only by the available RAM. The program is available for Mac OS X, Linux and Windows. It does not require installation: just run the executable. It means that you can safely try it and it will not change your computer in any way, except for a few settings stored from session to session.
For convenience the Windows version is available as an installer, but it simply adds a menu entry and it creates some file types associations to make its usage simpler.
The Linux version is also available as a .deb package: it simply adds a menu entry and icon for the program.

Starting from version 2.0.0 FSPViewer is color managed. If an image has an embedded ICC color profile that profile will be used to convert colors to the monitor profile.
This solves the problem with images created in the Prophoto RGB (and at a minor extent with the Adobe RGB) color space: if the viewer is not color managed the image colors look very wrong.

This program is freeware: you can use it for free but the source code is not available.