The presentations from the Cassandra Summit in San Francisco are posted here:
http://www.planetcassandra.org/blog/post/cassandra-summit-2013---use-cases-and-technical-presentations
I listed some of my favourites below.
First would be my own presentation: "Crossing the Chasm - SQL to NoSQL": http://www.youtube.com/watch?v=wIjvZRLUXsE
Here is an awesome presentation on debugging Cassandra: "In Case Of Emergency, Break Glass":
http://www.youtube.com/watch?v=7lY-33_Hn0Q
Titan is an up and coming graph database: "Distributed Graph Computing with Titan and Faunus":
http://www.youtube.com/watch?v=tYAWhBHQiXA
Spotify talks about lessons learned using Cassandra: "How Not to Use Cassandra": http://www.youtube.com/watch?v=0u-EKJBPrj8
