http://www.digitaldutch.com/arles/
Great tool, saves me many hours of work. But I still go back and "re-thumb" a few manually as needed.
Building a new gallery template can take me several hours. Selecting the full size images, dropping them into appropriately named folders, then using Arles to make the thumbs takes just a few minutes for each.
I made 10 new galleries on Saturday, using an existing gallery template, in less than 30 minutes.