Puppet was written by Luke Kanies after years of frustration as a systems administrator working with CFEngine. It's roughly three years old. It's mailing list has over 2,000 subscribers, and when I attended the recent PuppetCamp there were roughly 200 active participants in attendance. My very anecdotal estimate is that there are at least 30,000+ nodes using Puppet (My old consulting company was responsible for 6,500 nodes). Puppet is now maintained by Luke's startup, http://reductivelabs.com/ which took a $2m Series A round from True Ventures this summer. Puppet is written in Ruby/Rails, and implements it's own DSL for describing the configuration of your systems and applications.

Chef was written by Adam Jacobs of HJK Solutions, now http://opscode.com/. Adam was a vocal Puppet consultant, and wrote Chef in response to dissatisfaction around the handling of Puppet Bug #1010 (http://projects.reductivelabs.com/issues/1010), related to problems with idempotency initiated from Puppet's directed graph. Chef is maintained by Opsocde, whose CEO is Jesse Robbins from Oreilly and whose CTO is Adam Jacobs, they received a Series A round of $2.5m from Draper Fisher this summer.

Both are open-source, and sit somewhat in the same genre as CFengine, ISconfv4 (mostly abandonware), and BCFG2.