Some useful IRC commands that I constantly forget:
/msg nickserv register [password] [email] [additional info]
Register a nickname
/ignore JOINS PARTS QUITS NICKS
Ignore joins, parts, quits and nickname changes
/ctcp [nick] version
Returns details of the irc client being used by [nick]
/mode [chan] +o [nick]
Promotes [nick] to Operator status on channel [chan]
/mode [chan] +b [address]
Ban a user from [chan] based on his address. Format is: firstname.lastname@example.org
Example: /mode #irc +b *!*jp@*irc.com
Example: /mode #irc +b *!*@126.96.36.199
/mode [chan] +q [address]
As above but instead of banning a user, just make them quiet
/mode [chan] +c
Blocks colour codes from being sent to the channel
/mode [chan] +S
Strips colour codes – Only the originator sees his colours.
/mode [chan] +s
Secret – Hide a channel from the server listings. Channel can still be joined by anyone who knows the name.
/mode [chan] +i
Private – Users can only join by invitation (/invite [nick] [chan])
/mode [chan] +t
Restricts topic changes to operators only.
/mode [chan] +l [number]
Sets a maximum limit on the number of users allowed in the channel.
/mode [chan] +M
Only registered users may talk in the channel
/mode [chan] +R
As with +M, but also only lets registered users join the channel