I love this: The PocketMod
Nothing beats a folded up piece of paper.
The PocketMod is a small book with guides on each page. These guides or templates, combined with a unique folding style, enable a normal piece of paper to become the ultimate note card.
They have offline and online tools to walk you through the easy process of creating these small, convenient notebooks, although the online version crashed Firefox whenever I dragged around the pieces a few times.
I carry a pocket-size notebook with me everywhere, using it to maintain my To Do list, to jot down stupid ideas for blog posts, etc. I can’t see myself replacing my notebook with the PocketMod, but I still think it’s cool.