1. Click on the top (or left) ruler and drag it so it’s next to you horizon line (or other similar object).
   
2. Tools > Transform Tools > Rotate (or SHIFT+R).
   
3. Turn image using your mouse, slider, or angle box in Rotate dialog.
   

1. In the Toolbox:
   Set Interpolation to Cubic (best quality), or Linear (faster).
   Set Clipping to Crop with Aspect.
   
2. Press Rotate.
   
3. Image > Autocrop Image
   
4. Image > Guides > Remove all Guides
   

Gimp 2.6.11