Inspector tips
The inspectors are floating windows which contain information and controls related to what's selected in the main window. If you don't see them, click the Inspect button  in the toolbar, or pick one from the Inspectors menu.
You can show and hide an inspector's various sections by clicking the icons at the top of each inspector window. Hold Command while clicking to open an inspector section without closing the ones already open.
Each of the four inspector windows contains several different sections you can view; just click one of the icons across the top of the window to switch to that inspector, then click the icon again to hide the inspector. If you want to view more than one inspector in the same window, hold Command while you click their icons, or drag across several icons.
If there's a particular inspector section you find yourself needing to use quite a bit, you can lock it in the open position. Just double-click the section's button instead of single-clicking it; the button gets a green lock icon. As you show and hide other sections in the same inspector window, the locked section remains visible. When you're done, just double-click the button again and the lock icon goes away.
In all of the inspectors, you can click in a text field that contains a number, then press the up or down arrow keys to increment or decrement the number. Also, once you click a circular rotation control, you can keep the mouse button held down while moving the pointer away from the control to gain more rotational precision.
Any color well can have its color dragged to an object on the canvas or to another color well. When you drag to an object, you can drop the color on the object's stroke, fill, or text.
In places where you can enter measurements, such as in the Geometry inspector, values are shown in the current ruler units, or, if there is no unit scale, in the canvas units. You can, however, enter values in any units that are available in the ruler. As soon as you finish entering the value, OmniGraffle automatically converts it to the correct units.
You can also perform simple arithmetic and mix units. Add (+), subtract (-), multiply (*), or divide (/) numbers right in the input fields.