Here's my Wordpress Membership plugin review. I'm specifically talking about this one.
If you write a really popular wordpress blog, then I highly recommend this Wordpress Membership plugin.
What this plug-in does is basically let you create membership levels for your Wordpress blog.
For instance, you can make it so people have to pay to obtain certain access to your Wordpress blog. This could be useful in many different ways. Perhaps you are posting very valuable information in your blog that you feel people should pay for. This will fix that so you can get some money from your Wordpress blog. Perhaps you want to release software to people who pay per month, then you can set it up to release the software in those posts that users have to pay to access to. It's a pretty neat plugin.
Only downside is, the plugin costs a lot of money. But if you have a popular blog, you should get that money back from income you make through memberships. So check it out. :) I highly recommend this plugin!
Link to the plugin. :)