While there are a lot of good software solutions for storing sensitive data securely on your Mac, there aren't a lot of good ways to shield that data from the view of casual passers-by. If you store any of that information in text format, MacOSXHints.com reader djdawson has a neat way to keep it a bit more private:
I keep track of passwords and other sensitive information in a Rich Text TextEdit document, which I then store in an encrypted disk image. This way I can have access to this information when I need it, but it's secure until that time. However, I occasionally need to access that document while another person is at my desk, and I don't want to risk letting them casually see anything I don't want them to see.
My solution is to change the text color of the sensitive parts of the file to white and surround it with some obvious delimiters, such as periods. I can then just highlight everything between the periods to see the hidden text. For example, I might have an entry like this:
Server username/password: . Bob ./. $w0rdf1sh .
First, I select the text I want to hide and then change the color of the text to white (or whatever the background color of the document is, but it's almost always white). In TextEdit, that's means selecting Show Colors from the Format -> Font submenu (or hitting Command-Shift-C). I usually add some arbitrary white space between the periods and the text, so that the size of the space isn't a clue as to how long the password might be.
Note again that the white-on-white text is not the first line of defense: djdawson saves this file on an encrypted drive image, which is what really keeps his data safe. The text color hint is a second layer of security for people around you. It's pretty useful if you are working in a crowded environment, and want to be able to check one part of your document without subjecting the whole thing to wandering eyes.