Here's one we think you'll like (judging by requests on the recent survey) your very own WPMU DEV Multi Domain plugin.
It makes the process of adding extra domains to your site for you and your users to choose from incredibly easy, allows you to select whether they should be public (available to anyone) or private (super admins only), works perfectly with our existing domain mapping plugin and is completely cookie ready - so single sign on will work across domains.
And we';re looking into building in Membership and Supporter features to it now too.
Of course, also it's guaranteed to be supported, updated and work with the latest versions of WP, Multisite and BuddyPress.
So all in all, we think it's pretty good
But you know how you can make it better - have a play, let us know what you want / how you go and we'll improve it based on that too!
Here it is again: https://premium.wpmudev.org/project/multi-domains