Activity Monitor can be very useful.

Users and Groups

You can change your account and add new users by going to the users and groups window.

This site is getting a bit old but is good on security.

Keep your Mac clean

This is just one utility that cleans up your Mac.

Also check out these tips.

Apple have a security page

Apple put a lot of their security information on one page.

Time machine is used for backups.

You can save your entire Mac using Time Machine.

Check out this video for Time Machine on Snow Leopard.

Apple iCloud

You can back up to iCloud, but this itself presents some security problems. Check out this page.