Shotgun Farmers [Steam] really is a sweet FPS, I consider it a hidden gem since it doesn't have a huge following. It's been updated again with even smarter AI.
One of the problems you often find with indie FPS games, is that they don't end up with a big online following. Thankfully, Shotgun Farmers does have bots. This latest update has tought them how to jump, climb ladders and fall off roofs "correctly"—is there really a correct way to fall off a roof? They also have smarter targetting with the various weapons available.
There's been various other tweaks as well, like showing your killers name when you die, your profile will show kills with each weapon, shotgun shells are now popcorn and more. In addition, there's various bug fixes to make the experience better overall.
The developer is also working on more weapons and game modes, with them asking you to vote for which gun you want out of a "M6Bean", which looks like an assault rifle and a "Chillithrower", which looks like a flamethrower. Chillithrower definitely has my vote, sounds awesome.
See the full release notes here on Steam and vote for the next weapon to be added.
For those not quite sure what "where bullets grow guns" means: If you fire and your bullets hit the ground, a new weapon of that type will end up growing. It's really quite unique for an FPS and it actually works perfectly well.