David Karp's Founder Story
Chris got David to talk about how he came up for the idea for Tumblr. There's this part about 1:40 in where David talks about going from designing his tumblog to designing Tumblr. That's the aha moment. So great that they captured that.
I also like the part about 2:30 in where David talks about how the readers of the first tumblogs (which were mostly hacks) really wanted their own tumblogs and when Tumblr launched they could finally have them without having to do the hard work of hacking one together. That's the two step super distribution model. Get the power users to adopt something and then the people who follow the power users will adopt it too.