Thumbnail generator software for Mac OS X?

I would use ImageMagick for this. Here's a link to the OS X installation instructions. It is certainly not its sole purpose to generate thumbnails, but you can script a command for this based on numerous examples at the ImageMagick website. An example command would be:

convert -define jpeg:size=500x180  hatching_orig.jpg  -auto-orient \
      -thumbnail 250x90   -unsharp 0x.5  thumbnail.gif

Batch processing can be achieved using something like:

mogrify  -path thumbnails -thumbnail 100x100  *

These are only examples and you should check all the ImageMagick options relevant to the images that you will be processing.

support the mac developers, buy a copy of Downsize http://stuntsoftware.com/downsize/ or you could use automator...

ps. I'm NOT affiliated to Stunt Software.

I would use automator, so you would create a workflow that looks something like this:

First, you insert a get selected finder items action, so that the computer knows what files to work with.

Next, add a duplicate finder items action, because we don't want to replace the originals!

Next, we add a rename finder items action, and set it to replace text in the drop down box at the top. This is because the perevious step appends the word "copy" to the name of each file it duplicates, but I don't want them to say copy, I want them to be the file name followed by an underscore followed by the letters "sm", which stands for small, because these are the small images. So in the find field, we type a space then the word copy, so it looks like this Find: [ copy]. Then we type _sm in the replace field, so it looks like this Replace: [_sm]

Then, we add a crop images action, because that is our final goal: to crop them into thumbnails. Make sure the drop down box at the top says "to dimensions". Then, enter the dimensions of your desired thumbnails in the appropriate fields. Then, before you click run, make sure to set the scale before crop drop down box to Scale to Short Side, so it looks like this Scale before crop: [Scale to Short Side]. That way, the thumbnail will be a scaled down version of the image that fills the thumbnail box, rather than just being a tiny portion of the image.

Finally, find your images that you need thumbnails of in the finder, select all of them by either command clicking them or using the cmd+a shortcut to select them all, then press run!

There you go! You're all set with brand spankin new thumbnails.