I recently upgraded a laptop of mine from Intrepid Ibex to the Jaunty Jackalope beta, and wanted to get Rails development up to speed on there. However it seems that I fell into the usual pit hole when installing Ruby on Ubuntu that simply going…

sudo apt-get install ruby

…isnt enough, and doing so will produce this lovely error…

extconf.rb:8:in `require’: no such file to load — mkmf (LoadError)

So, to combat this, you need to do instead…

sudo apt-get install ruby1.8-dev

or for Ruby 1.9…

sudo apt-get install ruby1.9-dev

And then the error will magically disappear.