It's not free but it's the best one that I know of. Photoshop as you can reduce image size incredibly. Paint.net or Gimp are both free and are almost as good if you know what you are doing.
I second the IrfanfView recommendation. You can easily resize images, save them in tons of different formats, and a bunch of other cool stuff. Plus it loads really fast.
I use Advanced JPEG Compressor to reduce size of mu JPG/JPEG files, Its not free but its awsome sometimes the optimised image is just 1/3rd of the original size and there is no difference in quality.