Mac
For the most part all Macs ship with everything defaulted to a tight, secure setup as opposed to systems which ship with everything wide open.
Normal users, even on single user machines, are NOT automatically logged on as "root" or "administrator" or "super user" as are users on some machines. Normal users on a Mac must enter the root password to do any serious damage. Always stop and THINK about what you are doing if asked for the administrative password by any program or installer.
This is the DEFAULT operation of all Mac OS X systems. Max security unless set otherwise by the user.
Even though there are very few Mac viruses or spyware type things, it is still wise to run as if things were as bad as on the dark side.
The only security thing not turned on automatically by default at system installation is the firewall. Turn it on. Now. (See below)
Here's a scanner just in case anyone ever actually finds Mac spyware in the wild:
MacScan to scan for spyware, etc.
Other than that:
Set your built-in firewall to block all ports unless you specificly use them.
(System Preferences:Sharing:Firewall) Start the firewall. Once set, it will always start itself when the system starts until manually turned off.
Don't open any of the sharing functions unless you actually use them.
(System Preferences:Sharing:Services)
Even if you open up any of the above, if you only do it on a LAN, don't allow internet access.
(System Preferences:Sharing:Internet)
(FYI: The Firewall needs to be turned on, but otherwise, this is all set for max security AS SHIPPED.)