Hi, all!
A few minutes ago i downloaded an app for my Nintendo DSi called "My Notebook".
It cost 200 DSi Points ($2) via the online store, and just showed up for
download today.
It acts like a little notebook, and allows one to save many pages of doodles in
4 different colors. But the reason i'm telling you this...
It supports different paper styles, and one of them is classic grid paper. That
means you can draw your dungeons (or map out a current one) on the DSi. Take it
with you to your gaming sessions, and you've always got a handy mapping tool.
You can save your doodles, of course, and the notebook appears to support at
least 20 pages (each one DSi-screen in size).
Have fun!