Photoshop?
I have used Photoshop for stacking - very simple process.
Photoshop has a built in tool under File>Scripts>Load files into stack.
Once files have been loaded into the stack you can use Photoshop to blend the layers:
Edit>auto blend layers> select stack images, click OK.
Photoshop will then work its magic automatically determining the sharpest points of each image and blending them together.
Simples
Hope this helps
Mark

Thanks a lot- it also works in CS5
Lets get Stacking
Les