Zencoder
Zencoder is the creator of an open-source HTML5 video player, Video.js, built with Javascript, HTML, and CSS. Video.js plays video via native playback in all HTML5 browsers and devices, or through Adobe Flash when HTML5 playback is not available.
Hitbox.tv
Hitbox had about 6 million monthly active users in its later months. It has been announced officially use the HTML5 video player instead of using the Flash Video player.