Apple's keychain feature in OS X saves your passwords, keys, and certificates for seamless authentication to various system and Web services. In addition to storing keys, one of the features of the keychain is secure notes, where you can save important information like registration codes and product serial numbers, or authentication information for services that do not interface with the keychain. These notes will be securely encrypted along with your standard passwords, certificates, Safari credit card entries, and other information.
To make a secure note, you simply open the Keychain Access utility and choose "New Secure Note" from the File menu, or press Shift-Command-N.
This action will save the note to the selected keychain, which will default to your system's login keychain which is unlocked when you log in to your account. You can create additional keychains to store items separately; however, as with the login keychain, these will be saved as encrypted files on your computer.
This poses a small problem if you use multiple systems. Often people will have a home desktop system, along with a portable system and then perhaps a work system on a corporate network that they use.
If you use more than one Mac, even if it is an office system on a corporate domain, then in the latest versions of OS X, Apple has a new iCloud keychain feature that you can use to not only share passwords but any other features you use the keychain for, including secure notes.
To do this, simply log into iCloud in System Preferences, and then ensure that the "Keychain" feature is checked. This will set up an iCloud keychain that will be synchronized between your various Mac systems, and allow you to access and update its contents from any one of the systems.
When you do this, the iCloud keychain should be automatically populated with new or updated credentials you use, making authenticating to your various services relatively seamless on the various systems you use.
In addition, if you use the secured notes feature, then you can access them from your various systems as well. To do this, simply open Keychain Access, and you should see a new keychain called "iCloud" listed along with the "login" keychain and any others you have set up. Select the iCloud keychain, and then use Shift-Command-N to create a new secured note. Now copy in your desired text, images, and other information to the note, and save it. When done, the note should be synchronized to your other systems, and be available in the same location in Keychain Access.