- Photo Sphere Viewer by Jeremy Heleine
* good and popular viewer. Uses Three.js
- ThetaViewer by Atsushi Kokubo
* Viewer works great. Documentation is in Japanese.
- theta-viewer.js by Sho Hashimoto
* Animate multiple-Photos taken by RICOH THETA.
- Google VR for Web by Google
* Solid 360 image and video viewer that can be embedded in web site or mobile phone
- Pannellum by Matthew Petroff
* lightweight, free, and open source panorama viewer for the web
Also, over on the Unofficial Showcase, there are several companies that do virtual tours, worth looking through.
And there’s the List of THETA Apps, also with quite a few virtual tour options.