Zamzar - Free conversion online of many type of documents (images, text, video, audio...
iSpring Convert - Convert PowerPoint to HTML5 and MP4 video
Vertopal - Vertopal is an online platform for converting computer files to a variety of file formats.
Online Convert PPTX to MP4 - Online Convert allows you to convert any Microsoft PowerPoint 2007 PPTX file to MP4 easily.
Convertio - File Conversion in the Cloud
PPTX to Video - PPTX to Video is a simple but powerful mobile application that lets you convert a PowerPoint file to MP4 and play it on any video player.