There are many times when people may have items they would like to keep secured, which can include banking account numbers, software license information, or a secret Coca-Cola recipe. One way to keep these items secure is to save them to an encrypted disk image, but another option available is to use the OS X keychain's Secure Notes feature.
If you open the Keychain Access utility, you will see a Secure Notes section for each keychain; here you can add any bit of text, and it will be securely saved to the keychain. Here are the steps to do this:
Open Keychain Access
Select "New Secure Note Item" from the File menu, or press Shift-Command-N.
Name the item and enter your secret information in the Note field.
Click "add" to save the note to your keychain.
By default the new secured note will not be automatically accessible when the keychain is unlocked. This security measure will prevent people from accessing the note even if you leave your system unlocked.
To view or edit the note, double-click it in the list and you will be presented with information about it. Click the Show Note check box and you will be prompted for your password in order to see the note. After this, the note will be editable, and you can click the Save Changes button to save and lock the note again.
Though there are other ways to secure items in OS X including encrypted disk images, the keychain also has some benefits. These are that it is centralized, can easily be accessed through the system menu if you have the Keychain Status menu enabled (available in the Keychain Access preferences), and also is not a common method of storing information.
The downside to this is unlike Disk Images, which store passwords independently, the Secure Notes do not have their own passwords and are instead unlocked by the encompassing keychain password.
Correction: The keychain's password by default is the same as the login password and it will unlock when you log in, but if you change your login password the keychain password will not be updated, making the use of Secure Notes in it an even more secure way of storing confidential information.