I had never heard about the Master Method before, but found both the formula (for finding the big-O performance of recursive algorithms) and the derivation of it quite interesting. I also liked the graph algorithms, in particular the algorithm for finding strongly connected components in a directed graph (the algorithm uses the neat trick of reversing all the edges as one of its steps). The lecture on heaps was interesting – I studied heaps in my class at university, but I had completely forgotten about them. So there it really was a case of  needing a refresher (and heap sort now makes total sense too).