If you’ve ever worked in companies with 5-10+ servers and it was your responsibility to install new boxes, change some configuration files and install new software on many boxes you definitely know how painful this work is. Every time you need to change something on 3-5-100 boxes, you go there and make those changes. Most experienced of us used some weird scripts to perform some task on many boxes or used some stuff like dsh. Even with those tricks I’d never wish this work to anyone.
While I was working in Galt, I’ve asked our junior admin to check out puppet and try to use it on our servers. After a week of screaming he’s managed to install and configure it and said “Wow! This is what we definitely need!”. Since then he was using it and was happy 
. But, unfortunately (for me) I’ve never tried it and when I’ve changed my job and started doing the same old (but not good) things like dsh and for+ssh I’ve started thinking: “Stop! Try puppet – maybe it is what you need!”. And what can i say? Yes! This is what every admin needs! Today we’ve deployed our next web server (we do it pretty often) and it was done with puppet – pretty painlessly and quickly.
So, what is puppet? Let me quote their site:
Puppet lets you centrally manage every important aspect of your system using a cross-platform specification language that manages all the separate elements normally aggregated in different files, like users, cron jobs, and hosts, along with obviously discrete elements like packages, services, and files.
Puppet’s simple declarative specification language provides powerful classing abilities for drawing out the similarities between hosts while allowing them to be as specific as necessary, and it handles dependency and prerequisite relationships between objects clearly and explicitly. Puppet is written entirely in Ruby.
Basically, puppet is a tool which lets you describe whatever you’d like about your server(s) and deploy your configuration (configs, packages, files, directories, cron jobs, user accounts, ssh keys, etc) to any number of servers you need. You can describe some simple parts of your system (like “apache”, “nginx”, “rails”, “useful_gems”, “admin-ssh-keys”, etc) and then describe your servers using those terms (like machine X = apache+rails+useful_gems+admin_ssh_keys). When it is done, you’ll be able to deploy all this stuff with one command!
So, if you’re struggling with deploying some changes on many servers or, even, have only a few of them, I’d recommend to try puppet – it could save you a lots of time and nerves.
P. S. When you’ll try to use it – do not give up for a few days – it is hard to change your way of thinking and start writing correct and convenient scripts using puppet’s language.