Measure Twice, Look Once
If you need to measure more than one side of an object (for example, if you're measuring a box, and you need the height and width), you can measure both at the same time. First, open the Info palette under the Window menu (so you can see the measurements that the Measure tool generates), then get the Measure tool (Shift-I until it comes up) and click-and-drag it along the first edge. Release the mouse when you reach the end of the edge. Then hold the Option key (PC: Alt key), click on the end of the first line, and continue on in a different direction. You'll notice that another measurement line appears. Now, look in the Info palette and you'll see your two measurements listed under D1 and D2.