Jim Wang mentioned Social Locker. Great for boosting social traffic which hides content to be unlocked with a social share. I've used it to give access to our WordPress security ebook download link and will use this plugin more in the near future.

For plain old simplicity I love the GASP plugin, it massively reduces spam comments with a simple checkbox "tick if you are human".

I know thats 2 absolute favorite plugins, forgive me Jeff:)