Picture Resizer could be one of the simplest, yet most useful image utilities we've run across in a while. It's Windows only, but it's free and powerful.
All you have to do is download the Windows-only executable file to any directory. Then drag an image over the program icon. That's it. The image will automatically be resized. By default, all images will be resized so that they are 400 pixels wide. But wait, you say, I want to make my images 200 pixels wide, or 1600 pixels wide. No problem. Just change the program name from PhotoResize400.exe to PhotoResize200.exe or PhotoResize1600.exe.
If you want to resize a group of images, just highlight them and drag them all to the icon. Picture Resizer will convert the images and spit out new images (with the pixel width appended to the image names) in the same directory.
[Thanks JDeV!]
