Added in 1.5
/mode <#channel|nickname> [[+|-]modechars [parameters]]
Use to control a channel or nick, there are many modes depending on server, some common channnel modes are:
+b <nick!ident@host> - Ban a mask from the channel.
-b <nick!ident@host> - Unban a mask from the channel.
+i - Make channel invite only. (everyone needs to be invited through /invite to enter the channel)
-i - Disable invite only.
+l <N> - Set a user limit on the channel, when user count reaches N, no new users will be allowed.
-l - Disable user limit.
+m - Make channel moderated, only users with voice/channel operators can speak.
-m - Disable channel moderation.
+n - External messages from users not in the channel is not allowed.
-n - External messages from users not in the channel is allowed.
+o <nick> - Make a user channel operator.
-o <nick> - Demote a channel operator.
+p - Set channel as private.
-p - Set channel as public.
+s - Set channel as secret. (will no be shown in channel /list and so on)
-s - Set channel as not secret.
+t - Only channel operators can change topic.
-t - Everyone can change topic.
+k <key> = Set a secret key for the channel, only users who knows the key can enter.
-k - Remove the secret key, everyone can enter.