Hmm... isn't this a good idea for a wiki page?
I've
previously suggested using imagemagick. Though excellent if you need to batch-process dozens of files, it's rather demanding to master.
Gimp isn't the most user-friendly either, but you get decent results with it:
- Load image
- Rotate as necessary [From toolbox]
- Crop as necessary [Select rectangle tool in toolbox, then Image->crop to selection]
- Choose Tools->Color->desaturate to get B&W
- Choose Tools->Color->levels:
- Tweak black and white levels by:
- selecting the black or white eyedropper tools and click on a region of the image that should be black/white.
- and/or Use the sliders beneath the histogram.
- and/or click on 'Edit as curves' to get even more control.
I also like to use Filters->Unsharp to enhance the image.
There's also an excellent wavelet denoising plugin that can be installed which works wonders on grainy images.