Youtube offers the ability to caption uploaded materials. The automatically generated captions from Youtube are generally of questionable quality and will need to be double checked before being used in a class or as a promotional video. There are a number of great resources online that highlight how to use this tool. Some resources follow below:
TechSmith- Camtasia Studio
Camtasia Studio is a software product developed by TechSmith. They have a number of videos highlighting how to add captions to video utilizing their software. The videos below are only the ones produced by TechSmith. There is a host of other videos by the active user community.