Mrs is a lightweight but high performance implementation of the MapReduce programming model for parallel computation. It is optimized for computationally intensive programs, especially those which are iterative, though it is applicable to a wide range of applications. It is intended to be simple to install and use. It is written in Python, supports both Python 2 and Python 3, and has no dependencies outside the standard library. The name "Mrs" derives from the practice of referring to MapReduce programs by the title "mister" (e.g., "Mr. Wordcount").
I haven't tried cssh yet, and I'm sure it's great, but I would like to point out an inaccurate comment in the project description: PSSH does not spawn threads for all hosts. It runs with a fixed number of threads (1 to 2), and the number of ssh processes to spawn concurrently is configurable with a command-line option.
PSSH has an active mailing list and issue tracker, where any discussion of feature request is always welcome. It sounds like cssh has some great features, and collaboration is always welcome.