Comment by GrantS

Mean shift clustering, for one. This short PDF provides better insight than wikipedia. Does NOT require you to specify the number of clusters like in K-means:

http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/TUZE...

http://en.wikipedia.org/wiki/Mean-shift

Example python code using scikit-learn: http://scikit-learn.org/stable/auto_examples/cluster/plot_me...