Is there a repository for academic slides? Open access repositories like arXiv make it easy to archive one's papers at a stable place where everyone can download them.

However, in my field (computer science), many publications are conference publications, and they are presented by giving a talk (in addition to the paper). The slides of the talk can often be very helpful to understand the paper (or refresh one's memory, when one attended the talk). In addition, keynotes, tutorials, invited talks, course material, etc., are also many other reasons why academic prepare slides about their research.

Academics sometimes put their slides on their professional webpages (or on the websites of conferences, workshops, etc.), but these are not stable and often go offline after some years (e.g., when academics change institutions). That's a pity: one would imagine that there should be a website like arXiv but to deposit slides, keeping information about when the talk was given, which paper the talk refers to, etc.

Does such a website exist?
For computer science, the video lectures site is very helpful, VideoLectures 

This site includes a bunch of videos from well-known researchers in the computer science world from the top conferences in this field like KDD, NIPS and many others. Along with the videos of the presentation, you can easily download the slides material of the presentation (although you can't download the video files from this site). 
I recommend you to see this site.