Though here at Subscription Site Central, we mostly concentrate on using paywalls to convert free users to paid content subscribers, there are other options for paywalls -- advertising and brand recognition.Gigaom* this week reported on new service DoubleRecall which inserts paywalls in apps and websites and allows access to protected content to users who reads messages from sponsors and type in a few highlighted words to gain access.This isn't a totally new idea as there have been sites that ha...