I received the following error when installing Mongrel from the gem repository:

   1  marko@x61s:$ sudo gem install mongrel
   2  Updating metadata for 281 gems from http://gems.rubyforge.org
   3  complete
   4  Building native extensions.  This could take a while...
   5  ERROR:  Error installing mongrel:
   6  	ERROR: Failed to build gem native extension.
   7  
   8  /usr/bin/ruby1.8 extconf.rb install mongrel
   9  extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
  10  	from extconf.rb:1

The fix is to install the ruby development package:

   1  sudo apt-get install ruby1.8-dev