I intend to use chef or puppet to do administration (I'm thinking more of chef as it's younger and I get a better feeling about it).
In both home pages I saw there is an "enterprise edition" that costs money and I don't intend to buy anything. What would I miss in chef / puppet if I don't buy them?
What does chef offer that costs money exactly?
What does puppet offer that costs money exactly?
It was not so clear to me from their web site, as it's kind of obscure.
The paid versions offer more features (i.e., puppet offers an easier way to deploy en mass) and, in many cases most importantly, paid support. When running enterprise servers, having paid support to help you get setup is typically worth it--especially when you run into issues.
Typically, you won't go wrong with the free versions... its only if you need help getting up and running, or you simply have such a large infrastructure (and little experience with configuration management).
The software is free. The support for that software isn't.
I can only speak to my experience with Chef (which has been awesome). Chef is open source, you can use if for free, and they have good on-line documentation and wiki pages.
Chef has several 'flavors'. You can run the whole thing for free using Chef-Server (which requires you to set up the server software in addition to the client software on your system). Opscode also offers a hosted version (where they run the server for you) ... you can use the Opscode Platform for free for up to 5 nodes, after than you have to pay for the 'enterprise' edition.
Chef also offers a 'Chef-solo' version, that doesn't speak to a server, but I haven't found that to be very useful.
So, if you have less than 5 'nodes' (a node is a server you are administering) Chef is free. Or, if you want to go the route of hosting a Chef-Server.
Support has a fee, but the community is usually pretty good about offering advice.
I can't speak for Chef but I use Puppet where I work. There's an "enterprise edition" that comes with support but they also have a version that's totally free. Here's the link to the "open source" edition.
http://info.puppetlabs.com/download-puppet-open-source
I'm using puppet (the free one) to run a network of ~250 servers including VMs. It's great, but if I had it to do again, Puppet Enterprise might have been worth it. Time is worth money :) On the other hand, it looks like 250 nodes costs $17k annually!
In terms of chef, Opscode offers the open source version and a hosted version. The hosted version costs money, but they will provide a chef server for you and maintain it.
If you believe you can run your own chef server (and support it with online docs and forum postings), you don't have to pay Opscode for chef.
I would image Puppet offers the same sort of thing: a free product (without support), and a paid one (with support, possibly with hosting).
Most of the times you pay for support which these organizations provide. You can see the chart of comparison between puppet and puppet enterprise.
Having said that, several big organization are using puppet (not enterprise) and are totally happy with it. Being around for a little more time than chef, puppet has a bit mature community support and ready-made recipes free to use.
In short, feel free to go with non-enterprise version. There is a high probability that it'll fulfill your needs.