I'll add a couple into this , though (obviously) I may be a bit biased here
Lara - +a frigging million
This developer knows his WHMCS stuff, and keeps surprising me with better updates to his already excellent admin theme. It seems he's always testing stuff here.
WHMCS Notifications Extended (see link in sig)
I put this together out of a personal need. Since I'm on the road a ton, I needed a better way to get notified of tickets myself, and of issues. I then went and added client capability, and spent the better part of last year working on this. I can't say enough about it, but, again, I'm biased.
WHMCS User and IP Extended Control (again, see link in sig)
Again, I put this together out of personal need. Client came to me and said "Hey, Tom, I want to block orders unless they come from a certain country". Ok, done... Expanding on that, I added blocking admins unless they came from a certain country (or not), locking IP's after they hit a login failure limit, adding whitelisted IP's for admin (a-la .htaccess), with a script to automatically re-add them, if they had a valid session, blocking entire countries from simply visiting your page, logging client ip visits (all of them) and more.
This is a key addon for WHMCS that any admin should have in there. While it doesn't fully integrate with the fraud checks, it does work well, you just have to manually check this stuff.
Jetserver's CSF manager
This project seems to be somewhat abandoned, I may take that up as a hobby, but we'll see. The idea was to allow clients to search and remove their IP from CSF on your server, if you're using it. Great idea, just not sure it's been really developed over the past few months.
A no brainer for a WHMCS app developer, really, or, really any developer using php. it's pretty simplistic to get it into your code.