I think Source's NextBot system is one of my favourite AI frameworks in any engine. It's a very nice HFSM system with a lot of modularity.
And that SUCKS because for some reason it's not public. If you want to use it you need either a Source license or you need to use leaked code. I'm not sure what's stopping Valve from just dumping TF2's current NextBot base into the open. The base of the framework is entirely seperated out from the game-specific bot stuff so they wouldn't even need to release any of that. And it's great because you can basically just slot the NextBot base code into an existing Source project and it just works. Especially since the public 2013 base already includes the navmesh stuff.
It's especially a shame because what's currently there isn't entirely finished and there's a lot of stuff that could be fixed up and done to polish the entire system for better general use and community effort would help with that.
I've been quite tempted to make a NextBot-inspired HFSM AI framework for Unity for some time. I might get around to it at some point. Maybe integrate it into the base project stuff I use for everything.