Paid links

Well
theoritically paid link should be no follow but pratically they are not.There are lots of paid directories,sites etc who runs selling these.

You will also find a whole forum section dedicated to selling links in many big forums.

and if it do-follow then until google think otherwise(or manualy finds it) ,it would definately help PR,unless they redefine whole pr thing,so you dont need any proof for it.