I have actually learnt my lesson while creating a lot of thumbnails. One obvious example is here:

http://www.lbodnar.dsl.pipex.com/imaging/thumb.html

Notice how "ordinary" thumbnail cries for sharpening while "proper" one is sharp straight after downscaling.

I am using ImageMagick for batch processing. It has all I need - 16 bit processing, image manipulation, conversion, etc and it is free.