One of the more helpful things about living in the world of the Mac is the ease and convenience of creating disk images on the fly for archiving applications and documents. A couple of clicks of the mouse, and you've got a nice, neat little bundle of data—a DMG file—that you can store away, or even easily transport without whacking any file's Mac-ness—resource fork, Finder metadata, icons, and all that good stuff. Here's a tool that takes that ability and goes one better.
SecureFiles lets you create images, but it adds the ability to encrypt those images as well. Just tell it how big your image needs to be, give it a password, and start dragging files into it. It uses 128-bit AES encryption, so it's not likely that your data's going to be easily compromised. As a nice added bonus, if you guessed wrong when you originally established your image's size, you can compact it without breaking it.
SecureFiles is a Mac application. It runs under OS X version 10.4 or later.