http://www.digitaldutch.com/arles/
I second this. Arles makes the clearest, sharpest thumbs you'll find. Also does multi-folder batch processing, watermarking the pics, resizing, and will even create galleries for them if you want.
The program kicks ASS. The only gripe I have about it is that it doesn't do batch renaming (at least the version that I have); but I just use a small free program I found at Download.com for that. No biggee.
You'll love Arles. Try it.