Adding data to objects with the Note inspector
The Note inspector contains a field for entering text to associate with the selected object, and a table of custom data.
The note can be formatted as Rich Text, which means you can use all of the different font styles and colors that you can use elsewhere in OmniGraffle. When you put the pointer over an object with a note, the note's text appears in a help tag floating over the object.
You can use the custom data table to keep your own information about the object. Custom data is stored as key/value pairs: the Key is like a label for what type of information you are storing, and the Value is the information itself.
For example, imagine you have a diagram of a computer network, and you want to assign a model number to each component. Click the placeholder row or the plus button to create a new key/value pair. In the Key column, you would type Part Number, and in the Value field, you would type, say, A1181. This data doesn't have any effect on the way OmniGraffle works; it's just a way for you to store arbitrary data about objects in your diagram. To delete a row of data, click the 'x' button on the right side of the row.
You can also edit the note of a canvas itself. Just click the canvas in the sidebar (so that it becomes highlighted in your selection color), then use the Note inspector normally.
Notes can be found by Mac OS X's Spotlight search feature, in case you need to find your OmniGraffle diagrams that contain certain words.