<p>I'm going to rebuild my Linux box [yet] again.  I have to create a few user groups, user accounts and install my standard packages.  Until now I've just used the GUI tools.  I was wondering if anyone has any recommendations on writing a script to create users, groups and install standard packages after I do a minimal install of my latest Fedora build?  Sometimes I run Ubuntu so I'd like the script to be somewhat generic.</p>

Fedora and Ubuntu use totally different package managers, so you won't be able to easily do it in any sort of generic way.
In CentOS (which is RedHat Enterprise Edition with the serial numbers filed off, and so therefore pretty close to Fedora), we did this
using Kickstart files
.  These files have a simple syntax that enabled you to specify users, groups and packages to install, and even to script some custom stuff.