A basic panorama viewer that doesn't have lots of features, but it still can work for you depending on your needs as it does the job. Some settings include: field of view, rotation speed, damping and that's pretty much all. It provides ability to set a callback function on frame change, so some customization is possible.
Please note that its navigation mode is different from some other viewers: it uses what is called QTVR style when to navigate the image you need to press mouse button and move the cursor (and it can't be changed to "drag" style). Check it out below or on their demo page to see if you are comfortable with it.
Supported projections: equirectangular
License: MIT License
Distribution Method: Downloadable Library
Price: free
http://360-image-viewer-test.surge.sh/ (Drop your own equirectangular jpeg or png file to test)