Probably the most well-known in the free membership plugin space is s2member. (www.s2member.com) This is the plugin that I use and is my go to when I need to membership protect something. And honestly, what I'll be moving my site that's currently using paid memberships pro over to, I suspect. We'll see how that goes.
S2member, to me, is more of a framework. Where it doesn't provide the interfaces and step-by-step walkthroughs except for initial setup that Paid Membership Pro does, it offers a lot more features right out of the box. It also is a lot more flexible because it relies on shortcodes. This means that you can put membership signups and protections on many pages, not just the required pages that the plugin creates.
While I have yet to use support, that's because the forums are robust and I've been able to use a search engine with my question and find out the answer, and then implement it.
The one drawback, I feel, to s2member vs. Paid Memberships Pro is that the levels are pretty static. Free and then paid levels. The paid/premium version of the plugin allows you to set more than the initial 4 levels, and I haven't tried that, but that means that segmenting your free levels, for example, may not be as easy as it is with Paid Memberships Pro.
The price, however, is much cheaper for unlimited sites and, the thing I love about s2member is that if you do go with the paid version, which I have, it's a one-time charge. No yearly subscriptions.
The plugin integrates with most common software options and has a nice API system for those it doesn't. All in all, if I had to suggest a membership program to authors, though the learning curve might appear to be steeper than with other options, I'd tell them to go with s2member because there's a good chance it will do what they want with the free version, and the paid version will be much cheaper than others.