My favorite is in empty CD cases. You can use them from CD's that you now store in binders, or go to an office supply store and by a bunch in bulk for cheap. Take out and discard the plastic insert that holds the CD. I like to cut a piece of white cs to fit the front of the CD case, and then slip it in the front. This way I can see the images clearly on the front. I store them grouped by theme, and label the side with the theme so I can always easily find my stamps. For larger sets, like alphabets, I use larger empty DVD cases or the double-CD cases.

Variations include:

Slide the CD cases into a CD holder.

Stack the CD cases, label side out.

Sticking onto a transparency and sliding it into a page protector in a 3 ring binder.

Sticking onto a transparency and stacking inside drawers.