pssh and shmux have some customization issues, like inflexible IPs/hostnames specification, simultaneously spawning threads for all hosts (pssh), which is a problem with many hosts, or having to download/upload files with a separate command. cssh.py was written to resolve such problems. It has a fixed (configurable) length queue of active SSH threads, the ability to specify IPs/hostnames in 3 ways, configurable username, password, and SSH port per host, simultaneous download and upload of files/directories (using a built-in SCP implementation), again with configurable queue length, the ability to upload and execute a script with one switch, and many other useful options, like additional saving cmd outputs in a separate file for each machine (-d output_dir), or suppressing printing out hosts for which a given command produced no output (-b).
