Mahout has the implementation of the following clustering algorithms (as of release 0.9):
K-means clustering: This is available as both single machine and map reduce way
Fuzzy K-means: This is available as both single machine and map reduce way
Streaming K-means: This is available as both single machine and map reduce way
Spectral clustering: This is available only as map reduce way
Latent Dirichlet Allocation for topic modeling: This is available as both single machine and map reduce way