Tree Structure - Keyboard Traversal

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Got a question for some of the more web savvy guys out there, or
people who know about web standards.

We've got a tree-like structure in one of our apps, this is a web-
based application, off the intranet at work.  Anyway, we got this tree
and we've got some keyboard accessibility.  Mainly, you can traverse
the tree using the TAB key and shift-TAB to go up.  Pressing ENTER
opens a tree/node, and ENTER again closes.  We also have a menu, that
normally you would access using the mouse (right-click).  This menu
should be able to pop-up anywhere in the tree at any context (parent,
child, grandchild, etc).

What would be an appropriate key to assign this to?

Currently we've assigned the ESC key, but it doesn't seem very
intuitive to me.  ESC to me, has always been associated with exiting
or cancelling something.  Any thoughts?

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