When writing a ruby script I need an external library for Id3Tags. I tried to install it using gem (RubyGems is a packaging system -> Everything you need to know about).
It really took me some time until I could solved the problem:

Updating Gem source index for: http://gems.rubyforge.org
Building native extensions.  This could take a while...
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1

There is no package that has the name mkmf or something like that.
Thankfully, Google led me to the answer on RubyForge. For some reason, mkmf.rb is part of the ruby1.8-dev package, and initially I had not installed that since it is described as

Header files for compiling extension modules for the Ruby 1.8

A quick & easy

sudo apt-get install ruby1.8-dev

and everything trotted along happily after that. Don’t ask why I hadn’t already installed that development package.

EDIT: Maybe, it kinda seems like, it was not my fault not install ruby1.8-dev package, it seems to be a but in Ubuntu’s package depencies.