If you are referring to server idle time, any command that sends data through the server would reset it. A few examples are:
/ctcp
/say
/msg
/me
/nick
/topic
/mode
/whois
I'm sure you can think of more... I don't really want to list tons of commands. If you just want to keep yourself from being idle, just /ctcp yourself or /msg yourself every so often. Then you aren't bothering anyone and you're preventing yourself from being idle for a long period of time (if that's what you are trying to accomplish).
