JOIN
Syntax: /JOIN channel1,channel2, ...
Allows you to join channels.
Example: /join #chan1,#chan2,#chan3
The /join 0 command makes you PART all channels.

NICK
Syntax: /NICK newnickname
Changes your online nick name. Alerts others to the change of your nick
 

WHOIS
Syntax: /WHOIS nick
Displays information of user requested. Includes Full Name, Host, Channels User is in, and Oper Status
 

WHO
Syntax: /WHO mask
Who allows you to search for users. Masks include: nickname, #channel, hostmask (*.attbi.com)
 

WHOWAS
Syntax: /WHOWAS nick maxreplies
Displays information on a nick that has logged off. The <max replies> field is optional, and limits howmany records will be returned.
 

ISON
Syntax: /ISON nick1 nick2 nick3 ...

Allows you to check the online status of a user, or a list of users. Simple return, best used for scripts
 

CYCLE
Syntax: /CYCLE ch1, chan2, ...
This command is equivalent to sending a PART then a JOIN command.
 

MOTD
Syntax: /MOTD server
Displays the servers motd.

RULES
Syntax: /RULES server
Displays the ircd.rules of a server.

LUSERS
Syntax: /LUSERS server
Displays current & max user loads, both global and local.

MAP
Syntax: /MAP
Displays a network map
 

QUIT
Syntax: /QUIT reason 
Causes you to disconnect from the server.
Example: /Quit bye bye
 

PING
Syntax: /PING user
Sends a PING. Server sends pings on a timed
basis to check if users are still connected.
 

VERSION
Syntax: /VERSION server
Displays the version of the IRCd and other  info. 

LINKS
Syntax: /LINKS
Displays a list of all servers linked to the network
 

ADMIN
Syntax: /ADMIN server
Displays the admin info of a server.

USERHOST
Syntax: /USERHOST nick
Displays the userhost of the nick given.
 

TOPIC
Syntax: /TOPIC channel topic
/Topic #chan1 will display topic information
Example: Set new Topic:
/Topic #chan1 New Topic Text Here

AWAY
Syntax: /AWAY reason here
Marked as being away.
 

WATCH
Syntax: /WATCH +-nick1 +-nick2

HELPOP
Syntax: /HELPOP ?topic or !topic
 

LIST
Syntax: /LIST search string
< and > also * can be used
 

KNOCK
Syntax: /KNOCK channel message
Allows you to ‘knock’ on an invite only channel.
 

SETNAME
Syntax: /SETNAME

Allows users to change their ‘Real Name’  without reconnecting
Example:  
/Setname myNewName

VHOST
Syntax: /VHOST login password
Hides your host name.
 

MODE
Syntax: /MODE chan/nick mode
Lets you set channel and user modes.
 

CREDITS
Syntax: /CREDITS
Lists credits
 

LICENSE
Syntax: /LICENSE
Displays the License
 

TIME

Syntax: /TIME server
Displays the servers date and time.
 

BOTMOTD
Syntax: /BOTMOTD server
Displays the servers bot message of the day.
 

IDENTIFY
Syntax: /IDENTIFY #channel passkey
Sends your password to the services system to identify as the founder of a channel.
 

DNS
Syntax: /DNS option
Returns information about the IRC server's DNS cache.

USERIP
Syntax: /USERIP nick
Returns the IP address of the user in question. This will be shown encrypted.