I learned this really simple technique from my cousin, so many ThX (as always) to him, cause it saved me a lot of work.
1. The whole idea of this tutorial is to show how to straighten a crooked image. You probably don't have one, so open a random image and go to Image > Rotate Canvas > Arbitrary and enter a random value.
You could say that you can straighten an image without using the measure tool by going to the menu I just explained and enter a value, but then it's still guessing and your image will never be 100% straight. If you use the Measure tool, Photoshop will calculate the exact value needed to straighten your image. Yes, I know, another Buffy image, but then again, look at her for Christ' sake!
2. OK, so your image is crooked. Select the Measure Tool in your palette. It's hidden in the EyeDropper Menu. Now search in your image for a reference, in my image I will be using the window in the back. The frame has to be straight. If you have an image with text, it's even easier, raw a line (see next step) next to your text, cause your text has to be straight
3. Now click where you want to start on your image and hold your mouse down, your first click is the begin point, your second click the end, now you can release your mouse button
4. Go to Image > Rotate Canvas > Arbitrary, now you see that Photoshop has calculated and entered a value for you, click ok and your image should be 100% straight
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