In the tradition of year-end lists, the Knowledge and Information Systems journal has published a list of the top 10 algorithms in data mining, as compiled based on a survey given at the IEEE International Conference on Data Mining. Here they are (along with referenced to relevant wikipedia articles):
 C4.5 et al.
 Support Vector Machines
 Naive Bayes
Often times, these sorts of lists can seem rather contrived, but I pleased to see that Wu and colleagues have published a well-written--and thoughtfully referenced--survey of the Machine Learning and Data Mining literature, highlighted by pseudocode and reference equations. A great read!