The Opal outliner is now for sale as a release candidate (be sure to
use the $5 discount coupon). <http://a-sharp.com/opal/> -- it
requires Mac OS X 10.4, but Windows users can at least look at the
screen shots if they're curious.

It fully supports multiple selection. (Which, as Stephen alludes to,
does make the UI more complicated -- a number of people want the old
days of a toggling Expand/Collapse menu command. But a selection may
include both expanded and collapsed topics, and the program can't
really predict which you'd rather do.)
--

David Dunham A Sharp, LLC
Voice/Fax: 206 783 7404 http://a-sharp.com
Efficiency is intelligent laziness.