Plugin for using a menu across multiple sites?

I have a couple of instances where I would like to use a common menu among multiple sites. These are closely related sites that want a top navigation bar to be consistent, while having separate users, and separate sidebar widgets.

In the first case, I created a version of a theme with a hard-coded menu. The menu must be controlled through the back end, and I end up with an additional theme which is a pain when it comes to making minor changes or updates.

I am being faced with this issued again, and am wondering if there are any plugins out there that any of you know of that will allow a menu to be shared by multiple sites.

Any ideas are welcome! Thanks!