Well-designed object-oriented programs typically consist of a few key classes that work tightly together to provide the bulk of the functionality. As such, these key classes are excellent starting points for the program comprehension process. We propose a technique that uses web-mining principles on execution traces to discover these important and tightly interacting classes. Based on two medium-scale case studies — Apache Ant and Jakarta JMeter — and detailed architectural information from its developers, we show that our heuristic does in fact find a sizeable number of the classes deemed important by the developers.