Is there a repository for academic slides?
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.