Software to batch change jpeg resolution/size

We constantly battle with the amount of space pictures take up on our network.
Quite often pictures are needed for reference only, and can be stored at med/low resolution.
Does anyone know of a program that can quickly & cheaply reformat & save pictures at a preset resolution?
I understand there are a variety of picture editors that can open files and save in such a way, but I need something that can open an entire folder and reformat with minimal user involvement.
If anyone has any great suggestions I'd appreciate hearing them.

Thanks,
Alan Brown