'Pay with a Plus' - offering freebies in exchange for a Google +1 click.
Instructions and readme can be found here: http://thisisbensblog.com/pay-with-a-plus/
Examples can be found here: http://lab.thisisbensblog.com/paywithaplus/
Examples include instructions on how to set everything up, and the settings that can be used for each.
'Pay with a Plus' by Ben Pritchard is licensed under a Creative Commons Attribution 3.0 Unported License.