Among other things, version 0.5 of Gurkes Fotobatch brings multicore support for image resizing (view changelog). This results in an amazing speedup on multicore machines.

Download Gurkes Fotobatch v0.5 (zip archive, ~660 kB)
Works completely standalone. Successfully tested on Windows XP/Vista/7 (32 and 64 bit).
As always, you use the software on your own risk!