When I was using ubuntu I used a handy dandy shell script that my husband wrote to resize whole folders of photos. Since I still store all my photos on that machine I’m sure I’ll use it again, but I don’t seem to have the required elements installed on my mac to use that script.
Thanks to Automator (something I’ve previously completely ignored, but knew I didn’t want to delete) I can now do batch resizing on my MacBook. Mac OS X Tips tells you how. From the screenshot in that article you can see that there are a bunch of other things that you can do as well (cropping, rotating, thumbnails etc).
I thought Automator was going to be more trouble than it’s worth, but it’s really easy to use, with a simple, friendly interface!