I am going to assume you are asking this because you want to know when others are using your comp when you aren't around. Well, lets start off with the built in security function on any OS 9.x or OS X.
You can turn on the multiple users control panel and activate the Multiple user Accounts. This will require you to enter a password when you first boot the computer. There are a few apps and screen savers that do the same thing but this one is free.
Then there are a few programs that can record screen shots or key strokes. (like monitorer
This way when you log on you can see what others were up to while you were gone. Sneaky, yes but effective. Furthermore in some states not telling a person that this type of software is installed on YOUR computer is illegal. And using anything obtained as a result of the program (for let's say prosecution, divorce, etc.), will render the info inadmissible. Its considered along the same lines as wiretapping. Anyway that's just an FYI.....
If this isn't what you wanted to know then I recommend you expand on your post and detail what it is you want to know about your computer and what aspect of it being on you want to know.