/nick <new nick>
Changes your nick(name).

/me (Action Command)
This is used to speak in the third person. /me is replaced by your nick.
Example Usage: /me is tired.

/away <reason>
This marks you as being away from the computer. If someone messages you, they will be told that you are not there.
Example Usage: /away Eating dinner. I'll be back soon.

/away
A plain /away will set you back from away.

/msg <nick> <the message>
This is used to send a private message to someone. Unlike the query command, this will not open a chat window with that person.
Example Usage: /msg Jim Hello. How are you?

/query <nick> or /query <nick> <the message>
This opens a private chat window with a person. If you use /query <nick> <the message>, when the window opens, the message will be sent and appear in the window.

/dcc chat <nick> (Direct Client to Client connection)
Unlike the query chat, this type of chat is done thru a direct connection to the other person’s computer. It is not relayed thru the IRC server and as such is more reliable and secure. Please note that this will reveal your IP to the person you are chatting with - even if the IRC network masks your hostname.

/whois <nick>
Gives you some basic information on a user.

/join <#channel>
Make you joins the specified channel.

/part <#channel>
Leave the specified channel.

/quit or /quit <reason>
In a normal client program, this is the same as closing the program. In a java client, it will just disconnect you. When you add a reason after quit it will send it to the channel.
Example Usage: /quit I must go to sleep now. Bye!

NOTE: Some of these commands are available by right clicking on a nickname.