You can get a list of all available commands:
/help

To tip another player type:
/tip <username> <amount in BTC>
Example: /tip tiCeR 0.1

To talk private to another player type:
/w <username> <message>
Example: /w tiCeR hello.

or

/pm <username> <message>
Example: /pm tiCeR hello.

To reply to your last private message type:
/r <message>
Example: /r message.

To send a message to all mods that are online type:
/mods <message>
Example: /mods There is a team on faucet playground

To ignore a user in the chat type:
/ignore <username>
Example: /ignore tiCeR

To check if a user is online type:
/online <username>
Example: /online tiCeR

To reconnect to the chat type:
/reconnect