Jakes node as tabs is an excellent navigation add-on allowing for tab management the most useful add-on on xenforo imo. FREE
Not sure if your worried about social features that adds to the social aspect of a site
Post ratings - by Dark Immortal: an extension on the likes system that allows you to give more than a like. PAID Xenmoods - I find a nice feature that members always like (though I'm not sure what your site entails) but members tend to like these. FREE Notifications - By Chris Deeming: Very useful notification for your guests members and has tons of options and a bargain a very useful and versatile add-on. PAID Bookmarks by Syndol: Again a useful bookmarking manager for members to keep track of their fav posts. PAID
There's also a range of other good add-ons but it depends what kind you are looking for, kind of site you have and really how you operate. But the above are ones I find useful Node as tabs is a must no site should be without this.
I also put Nodes As Tabs by Jake as the top addon.
Syndol's Conversation Essentials - it shows who the other person is on the Conversation list plus lots of other useful controls. FREE and PAID versions. Stay Logged In - not sure who does this but it's a small but vital contribution to civilisation - it automatically default ticks that littler box that keeps you logged in FREE. Library by Waindigo. FREE. Users (permission by usergroup) can create Article pages exactly like writing a forum post, using the bbcode editor. Pictures/ photos so can create a gallery. Articles paginate and can link to a forum or thread to discuss. Articles collect in a Library with preview snippet. Etc.