I find the Palm PDA have the smallest and lightest devices. This is important to me since I like to carry a PDA with me everywhere. I use the Palm Vx that is now an old model, but it is very small and light and the batteries last a long time. The screen is only gray scale but you can find a Palm Vx for dirt cheap on Ebay. It uses OS 3.5 that still runs all the Palm software and it has 8 MB memory that is still plenty.
My 2 cents, Ed