Pannellum

Pannellum is small, fast, easy to use. Though pretty basic, it has an interesting feature — multires panoramas (ability to use images with different resolution depending on zoom for single panorama). It claims to work in all modern browsers with WebGL support. Mobile browsers are not officially supported, so they may or may not work properly.

Supported projections: equirectangular
License: MIT License
Distribution Method: Downloadable Library
Price: free