`require': no such file to load -- zlib (LoadError)

/usr/local/lib/ruby/site_ruby/1.9.1/rubygems/package.rb:10:in `require': no such file to load -- zlib (LoadError)

So you get the above error when attempting to install a ruby gem in ruby 1.9+?

First: make sure you have libzlib installed:

sudo apt-get install libzlib-ruby

Then go back to where you unpacked ruby 1.9 and rebuild:

./configure
make
sudo make install

done!