Obscurity seems just like a generic Mac OS X folder (official download link is down; alternative download available here). Simply place the 'folder' anywhere you want, and anyone snooping around your stuff deciding to double-click it will unknowingly arrive in an empty mock folder. However, if you right-click the 'folder' and choose 'Show Package Contents' from the contextual menu, a completely different folder will show up ready for you to hide all those intimate movies, pictures and old love letters in. It even hides all your secret documents from Spotlight and those pesky 'All Images', 'All Movies' and 'All Documents' Smart Folders in Finder. You can rename and/or duplicate Obscurity to your heart's content creating as many custom secret folders as you need. Obscurity doesn't modify your files or the operating system in any shape, way or form making it the safest way to keep your stuff away from curious eyes.
On an interesting related note, Obscurity's developer, Brynjar Saunes Bye, is quite the polymath. Not only has he published several software utilities for OS X, including Hypnotize (a collection of timers for your Dock telling the movie to shut up and your Mac to shut down when time is up) and Functional (a collection of utilities bringing the basic Mac OS X functions from the Apple menu to your Dock), but he's also a medical doctor, graphic designer, TV producer, neurolinguistic programmer, musician, and actor!