An Ubuntu astonishment
I just discovered that my Ubuntu test machine (running Dapper, not the current bleeding edge), installed with a base level of server packages, does not have cron installed.
It does, of course, have the /etc/cron.(daily weekly) directories, and cheerfully optimistic packages have populated them with various things that they no doubt expect to get run every so often in order to keep the system working smoothly.
(Evidently none of them actively depended on cron, or it would have been installed. Perhaps this is a bug. Fortunately exim4 did depend on cron and I actually read the list of packages that were going to be installed as dependencies, so cron is now explicitly listed as part of our additional core packages.)
Sidebar: a fun Ubuntu dependency chain
As an experiment, I started to install eximon4 (an X11 based Exim monitoring program). To my surprise, apt told me that it wanted to install dmidecode, which struck me as a rather absurd thing for an X11 program to require.
Except it doesn't, not directly. eximon4 requires libx11-6 (the main X libraries), which requires x11-common (the base X infrastructure), which requires laptop-detect (tries to detect if you're running on a laptop), which reasonably requires dmidecode.
(The dodgy hard dependency is x11-common to laptop-detect; I would rather not import laptop detection code on servers just so that I can run X clients. Current Debian packages have moved this dependency to the X server, which is more rational.)