Updated 23 June, 2019

All commands are typed in in-game chat and begin with a  /   Commands have access levels and won't work for you if your level is too low.
Note:  There are actually commands missing from this list and a few commands are deliberately undocumented.  Those commands don't do anything profound.  They are there for you to discover in the code as a reward for reading it.

Fun Commands:

/bounty {optional player name}
/view bounty {optional player name}
/view bounties
See the player kills and current bounty on a players head or on all players currently on the server.

/gimme gimme
/gimme peace
Make gimme messages appear in public chat with /gimme gimme or as private messages with /gimme peace (with some exceptions).

/fix gimme
Force the bot to rescan the list of zombies and animals.

/reset gimmearena
Cancel a gimmearena game in progress.

/gimme raincheck {seconds}
Set a time delay between gimmes. The default is 0 seconds.

/gimme reset
Reset gimme counters for everyone so they can play gimme again. The bot does this every 120 minutes automatically.

/gimme on/off
Enable/disable the gimme game.

/gimme zombies
/gimme no zombies
Enable or disable zombies as gimme prizes.

/gimme reset time {number} (In minutes. Default is 120)
Reset everyone's gimme counter after (n) minutes.

While in any location with beer in its name, players can grab a beer (or a lot).

Don't do it! :O

/place bounty {player name} {cash}
Place a bounty on a player's head. The money is removed from your cash.

Play one gimme - win a prize!
Gimme cannot be played within a location unless it is pvp enabled.
Gimme cannot be played inside a player base.
Prize may contain nuts. If a rash develops, see your doctor. Keep away from small children. The bag is not a hat.

Get kicked out of the server with a random message.

/quit {message}
Get kicked out of the server and have the bot say your message in game chat.

/gimmezombies or /gimmehell or /gimmeinsane or /gimmedeath
Play a special gimme game in a location called arena. You and anyone with you will get 4 waves of zombies to fight.
Select one of 4 games of increasing difficulty (more zombies, faster spawns, harder zombies).
Admins or arena players can cancel the game with /reset gimmearena
Zombies are randomly distributed between arena players. Any players more than 5 blocks above the arena floor (or under it) are specators and don't get zombies.
Some useless crap is supplied at the start.

/doge mode or /doge on/off
But what does it do!? Play and find out xD