Python packages for clustering:

SciPy - www.scipy.org (kmeans)
python-cluster - sourceforge.net/projects/python-cluster
PyPR - pypr.sourceforge.net (kmeans, gaussian mixture models - GMM)
scikit - scikit-learn.org/stable/modules/clustering.html (kmeans, GMM, DB-SCAN, hierarchical)

examples:
http://www.ar.media.kyoto-u.ac.jp/members/david/softwares/em/

Learning resources:
http://en.wikipedia.org/wiki/Cluster_analysis (large overview, missing some newer research)