Is your image crooked? Not a problem, there is a quick fix for that. Switch to Measure tool - under the droplet (Shift-I) and draw the line along your lines in the picture, which should be vertical or horizontal. It could be the horizon line or a wall of a building - simply mark the line that should be straight. And the go to the main menu - Image, Rotate Canvas, Arbitratry. The angle will be already there. Press OK and here it is - your image is straight, you just have to crop off the unwanted areas.

Have you ever tried to precisely crop an image when there are just a few pixels that you want to get rid of? The crop tool just snaps to the document border or to the layer border. Sometimes I just zoom in to make the difference much larger and then adjust the crop area.
But there is a better way - Hold Ctrl key (or Command on Mac) and adjust the crop area to precisely crop your image - down to a pixel.
As well when you use your crop tool you can easily move the crop area with the arrow keys and zoom in/out with Ctrl/Command "+/-" keys.