Secure your Mac: Crouching user, hidden folder
Here are a few very simple steps you can take to enhance your Mac's security - not exactly the U.S. Government's Advanced Encryption Standard (AES) algorithm, but enough to thwart many end-users.
Create a guest user account
Occasionally, someone will ask me, "May I quickly check my email on your computer? Just real quick..." (often these are Windows users who need an excuse to play with the MacBook Pro). "Sure," I say, but before I hand over the reigns, I switch to my guest user account. Here's how you can create one.
Click System Preferences, then Accounts
Click the small "+" icon at the bottom of the window
In the sheet that appears, set up your guest account. I got fancy and named mine "guest," with a password of "guestpw," but you can choose whatever you like.
De-select "Allow user to administer this computer."
Now, when someone "borrows" my computer to check their mail, they see an empty home folder, blank email client...the works. Even the rude ones who try to quickly peek won't find anything.
Create a hidden folder
Mac OS X automatically hides a folder that begins with a period. We can use this to our advantage and create a secret folder. However, it's not as easy as creating a folder in the Finder and naming it ".MyPrivateStuff." But it's not difficult, either.
Open Terminal. By default, you're in your Home directory
Create a new folder with a "." as the first character using the "mkdir" command, like this: mkdir .MyPrivateStuff
Hit return. You're done!
So how do you access that folder from the Finder? Navigate to your Home folder (open a new window and click the little house in the left sidebar). Now, select "Go to folder..." from the menu bar. Enter the name of the folder you created, including the period (in my case, .MyPrivateStuff). Presto! Your hidden folder appears in the Finder window. Now populate it with your "secret" stuff (financial in nature, of course).
One caveat: The Finder "remembers" the last folder you visited with this method, so be sure to "Go to..." a benign folder before you walk away.
Combine the two tips
If you really want to get fancy, combine the two tips: Create a new user account that does nothing but store your secret folder(s).
Again, this isn't military-grade security, but simple techniques that you can use as an extra layer of protection for some of your stuff. Good luck.