No worries – I’m not affiliated with or receive any commission from these plugins. I’ve just been happy with their functionality and usefulness and have used them on several projects.
ThreeWP Activity Monitor
Want to know what’s happening on your site? This little plugin logs *most* if not all possible activities that can be logged. The only thing better would be a server log. Can also be useful in identifying which user changed what and when.
All In One WP Security & Firewall
Very user friendly, walks you through some good security protocols, provides some degree of brute-force attack protection, provides meaningful firewall options (and explains them) and then it even provides easy functionality to make backups of your .htaccess, wp-config files and database. The only downside is that the firewall options are Apache only (although one can certainly borrow the ideas in their firewall settings and translate it to the server of their choice).
Pods Framework
See also http://pods.io/ for documentation and forums. This is an extremely powerful plugin for extending WordPress. Everything from Custom Post types to…well… custom data objects that aren’t even a part of WordPress. You can use this to bridge WordPress and other data sources to create some very powerful functionality. When I first found Pods the documentation was poor. But now, it’s pretty decent. You do need to have an understanding of PHP though and Pods is definitely not for beginners (except using it for Custom Post Types). But if you need to extend WordPress or build a bridge between WordPress and another application, I’d recommend taking a look at Pods.