I'm not sure why it doesn't seem to be more well known, but CreditKarma has allowed free filing of taxes (incl. capital gains), and was recently acquired by Intuit. Last I read they were under investigation by the Department of Justice because of the implications they were eliminating a competitor.

Comments: https://news.ycombinator.com/item?id=24038559

I've used it for the past 2 years, and it has worked great for me, albeit I don't have very complicated taxes. I'm actually afraid of what happens if the acquisition goes through, definitely expecting the free tax filing to disappear soon after that occurs.