QRCodeMagick is an easy to use gem that makes generating QRCodes and rendering them to images super simple.
QRCodeMagick is very simple to use. There are two basic methods: draw and draw_image. Below is an example script that show how both methods work.
require 'rubygems'
require 'qrcode_magick'
# Generate a RMagick drawing for a qrcode
drawing = QRCodeMagick.draw 'some url'
# Generate a RMagick drawing for a qrcode with some options
drawing_with_options = QRCodeMagick.draw 'some url', :scale => 8
# Generate a RMagick image for a qrcode
image = QRCodeMagick.draw_image 'some url'
# Generate a QRCode and write it to disk
image = QRCodeMagick.draw_image 'some url', :scale => 8, :write_to => 'qrcode.png'