The best free one I have found:
Batch Picture Resize
Best in my case is because I needed a way of mass resizing across multiple directories with one click; I have a 1TB USB HDD plugged in to my NAS, one partition of which is devoted to making my photo and video collection always available. When my wife started taking advantage of this to take photos for e-mailing I realised that both e-mailing constraints themselves and fast next-next-next-"ah, that one" browsing over WiFi counted against 5MB (and more) photo sizes.
The resizer I have linked allows me to drag a root Windows Explorer node on to the resizer icon on the desktop, release, and have all the Jpegs in that folder and child folders resized even if the folders contain other file types. It is fast, too.
If you try it, note that the icon name denotes all the features it uses, including the size that you desire the processed pictures to have. Neat.