I've been using MongoDB to rewrite the activity feed on one of my apps. I wouldn't use it for everything though (not just yet anyway).<p>Having said that, the guys behind Harmony (<a href="http://get.harmonyapp.com/" rel="nofollow">http://get.harmonyapp.com/</a>) use MongoDB for everything, as far as I know: <a href="http://railstips.org/blog/archives/2009/12/18/why-i-think-mongo-is-to-databases-what-rails-was-to-frameworks/" rel="nofollow">http://railstips.org/blog/archives/2009/12/18/why-i-think-mo...</a>