Standard commands

/me <message> - Sends a message as an action, for example calzoneman gets a snack.
/sp <message> - Hides a message in a hover-to-expose spoiler box.
/afk - Toggles your AFK status. You will automatically be marked not AFK after sending a chat message or voteskipping the video.
Moderator commands

/say <message> - Sends a message in big red text.
/poll <title>,<option1>,<option2>,... - Opens a new poll (this is from before the New Poll button was added).
/hpoll <title>,<option1>,<option2>,... - Same as above, but hides poll results.
/mute <user> - Mutes a user from sending chat messages.
/smute <user> - Shadow mutes a user. They will still receive their own chat messages, but nobody else will and they will not appear muted except to moderators.
/unmute <user> - Unmutes a user. This will unmute both muted and smuted users.
/kick <user> [<reason>] - Kicks a user, optionally providing a kick reason
/kickanons - Kicks all anonymous users from the channel
/ban <user> [<reason>] - Bans a user by name, optionall providing a ban reason
/ipban <user> [<reason>] - Bans a user and all IP addresses they've used in the past month, optionally providing a ban reason.
/ipban <user> range [<reason>] - Bans an IP range (/24 block for IPv4 or /64 block for IPv6)
/ipban <user> wrange [<reason>] - Bans a wider IP range (/16 block for IPv4, /48 block for IPv6)
/clear - Clears the chat buffer.
/clean <user> - Removes all playlist entries from the given user.
/cleantitle <title> - Removes all playlist entries that match the given title.
/d <message> - Calls a drink with the given message
/d# [<message>] - Calls # drinks, optionally with a message. # can be positive or negative.