What if your Mac is stolen? How much personal info do you have just sitting there on those hard disk platters? My Mac has Tax info for the past seven years, copies of bank statements, credit card PINs, and receipts for almost everything I have bought (see this post about archiving it all). It's rather disturbing to think of all that info falling into mischievous hands.
Up until about 4 months ago, I was using Apple's FileVault (see this post). This is a great way to keep things secure. It essentially encrypts your data into a disk image that is unlocked on boot up. The problem comes when you want to migrate everything to a new disk, like I tried. FileVaults can not be migrated. You must install a fresh system on the new disk, mount the Apple FileVault disk image, and drag everything over manually. No thanks.
I quickly switched back to using the excellent software Knox from Marko Karppinen. Knox also uses encrypted disk images to keep out prying eyes. They can be opened on any mac (given the correct password), which makes them easily portable. Knox also runs as a system wide extension that will auto-mount an image on boot-up, backup encrypted disks, and (this is the big one) the disk images are searchable by Apple's Spotlight. Basically, it's like just keeping a separate secure drive with all of your sensitive data on it. Sure, you can do most of this with Apple's disk utility, but Knox really makes it all seamless and easy to maintain.