<p>You can see also <a href="http://arxiv.org/pdf/cond-mat/0308217" rel="nofollow">Finding and evaluating community structure in networks</a> by Newman and Girvan, where they propose an aproach for evaluating communities in networks(and set of algoritms based on this approach) and measure of network division into communities quality (graph modularity). </p>
Distributed hierarchical clustering
<p>Are there any algorithms that can help with hierarchical clustering?
Google's map-reduce has only an example of k-clustering. In case of hierarchical clustering, I'm not sure how it's possible to divide the work between nodes.
Other resource that I found is: <a href="http://issues.apache.org/jira/browse/MAHOUT-19" rel="noreferrer">http://issues.apache.org/jira/browse/MAHOUT-19</a>
But it's not apparent, which algorithms are used.</p>