Rost Lab/Bioinformatics Group, Technical University of Munich, Germany
Our cluster includes over 30 worker nodes and several servers (http://rostlab.org/ganglia).
We chose Debian 'stable' because of its low maintenance need due to it being unchanging for the long lifetime of stable releases. Also, we find the way the distribution is build (configuration files, packaging policy) very convenient and well-thought-out. Thank you for the great distribution.