SSH::Batch is a simple command line tool, written in Perl, that allows you to run shell commands over SSH across multiple servers. These days it seems most people turn to Puppet / Chef / Ansible for that type of thing, but sometimes your needs aren't that complicated. For that, SSH::Batch fills the gap nicely and it's really simple to get started.
Have you ever been working on a website and needed direct access to the database, but couldn't get access without using something like phpMyAdmin? SSH tunneling can solve this common problem and a whole lot more.