Help with making designs over the internet

I'm just trying to find a program or something that would help me and a friend with a project we have.

We plan on designing our own game (somewhat simple, highschool graduates bored over the summer).  Unfortunately, we're some distance away, so I'm wondering if anyone knows of a good program where we can make designs/drawings simultaneously.  Pretty much we're looking for a multiplayer paint program.

Any help would be appreciated.