Mac: Several applications can resize a bunch of images, but it often takes some time and fiddling. Like with most Terminal commands, it just takes a little string of text to get your images down to size fast.
MacLife points out this helpful command line trick. Simple navigate to the directory you want in the Terminal (using the cd command) and typing the following (or variation of it):
sips -Z 640 *.jpg
So what's happening? Well, "sips" is the command being used and -Z tells it to maintain the image's aspect ratio. "640" is the maximum height and width to be used and "*.jpg" instructs your computer to downsize every image ending in .jpg. It's really simple and shrinks your images very quickly. Just be sure to make a copy first if you want to preserve the larger size as well.