Use gimp to easily straighten crooked photos!
Here is my random photo which is slightly crooked to the left.
First choose Tools > Transform Tools > Rotate
Then you can simply adjust the angle to straighten your photo as desired. If needed, you can also adjust the center pivot point.
I turned on the grid to help straighten my photo. This photo just needed a slight nudge clockwise.
At this point you may want to crop your image to cut out the uneven edges, if desired.
Related and sometimes useful is the Perspective Tool.
This is likewise located under Tools > Transform Tools > Perspective.
If you have ever wanted to simply grab an image with your mouse and drag parts of it up or down, this will do it.
Below is my somewhat silly example, but you get the idea of what is possible.