FraudRecord is good add-on but its not free anymore it costs money so its not really worth it.
Even when v2 comes out, it'll be free.
Adding to this list, my own stuff (mostly things I developed myself):
User and IP Extended Control
Ban users, logins, admins by country, just allow orders from X country, just allow admin logins from X country, tie IP's directly to admin logins, etc.
WHMCS Notify Extended
Get slack, pushbullet, telegram, hipchat, ryver , pushover, campfire, SMS notifications sent to both staff AND users on a variety of things, and have notifications sent when tickets have been 'idle' too long (dependent on department and status).
Not stuff I developed
WHMCS Licensing Addon
Lara, the OTHER WHMCS admin theme
Fraudrecord as mentioned
You need to know the longevity of those making the addons, you have great companies like Modulesgarden, Zomex, Whmcs Services who just deal in modules and templates and issue regular updates when WHMCS issue new or updated versions.
I'd second this. Make sure you know who you're dealing with, and hey, even if they appear to be offline, send 'em a ticket. There are those of us who just simply don't like forums any more.
Avoid the
hell out of Modulesgarden though. These guys are the worst of the worst to work with. Just ask anyone who's ever had to have them solve something
urgently. It takes weeks to get something done right