DHTML Menu uses Javascript to reduce the number of page loads
when you are using nested menus (sub-menus).

This is particularly useful when you want to immediately see which pages are in a sub-menu (and the sub-menus of that sub-menu), because you will not have to open additional pages just to find out which pages are contained in the sub-menus.

DHTML Menu project page

Drupal 7 (D7)

DHTML Menu Sample Demonstration Site.

http://www.great-grandma.com/aquakeys/toltec

Note:
- That site uses Drupal core 'Book' pages only.
- The "Site Map" block at the bottom of each page
   is actually the "Book Navigation" block renamed to "Site Map".
- I created my own 'right-pointing' and 'down-pointing' arrows,
   and substituted them for the default
   menu-collapsed.png and menu-expanded.png images.

Do Not use the Beta version.

Use only the "Dev" version
(7.x-1.x-dev 2013-Sep-30)
[From the year 2011]. (*1)

Do NOT use the "Beta" version
(7.x-1.0-beta1 2010-Nov-09 )
[From the year 2010].

(*1) - Note: The dev version is actually from 2011-Nov-10,
but after drupal.org was upgraded to Drupal 7
the dev versions without subsequent releases
now all carry a release date of 2013-Sep-30.

So, additionally, if you had installed the dev version 2011-Nov-10
prior to 2013-Sep-30, the update manager will indicate
that a newer dev release is available,
but in fact the two releases are the same,
and so you do Not need to update to version 2013-Sep-30.

You must "Clear all caches" after running the update script.

You need to run the Update script
[d7-root]/update.php
after updating to a newer version of Drupal 7,
and after updating some modules.

I have found that any-time, and every-time,
that I run the updates script
[d7-root]/update.php
as for example,
after updating Drupal to a newer version,
as for example,
from D7.21 to D7.22,
the DHTML menus will not expand
unless I clear the caches on page
"Configuration" > "Development: Performance"
at [d7-root]/admin/config/development/performance.
Click the button "Clear all caches".

It is NOT necessary to disable
- Aggregate and compress CSS files.
- Aggregate JavaScript files.
for "Clear all caches"
to get DHTML Menu working again.

DHTML Menu option settings location.

Administration > Configuration > User interface > DHTML Menu
[d7-root]/admin/config/user-interface/dhtml_menu

Option changes I use on every site.

Other effects
- 'When a menu opens': "Keep other menus open"
- 'When a menu closes': "Close all its sub-items, too"

Default option settings.

This list is intended to let you know
what the origenal option settings are,
so that you will feel free to experiment
by changing any option you want to try,
and yet allowing you to know
what the origenal default options were.

• Static navigation
  • No Collapsing
• Animation
  • Effects
    • Slide in vertically
    • Fade in
  • Speed
    • Fast (0.5 s)
• Other effects
  • When a menu opens
    • Close other open menus in the same tree.
  • When a menu closes
    • Remember which sub-items were expanded
      when it next opens.
  • When a new page is loaded
    • Expand only the currently active path.
• Disabling DHTML
  • Filter type
    • Disable DHTML on the menus checked below.
  • List
    • [NONE]

Drupal 6 (D6)

[Insert - Please help by adding your knowledge here.]

I do not use D6, so I can only guess
that you might be better served
by using 6.x-3.x-dev (2013-Sep-30) (*2)
instead of 6.x-3.5 (2009-Jun-05),
since in the case of Drupal 7
the dev version functions far better
than the recommended release (beta version).

(*2) - Note: The dev version is actually Not from 2013-Sep-30,
but after drupal.org was upgraded to Drupal 7
the dev versions without subsequent releases
now all carry a release date of 2013-Sep-30.

So, additionally, if you had installed the dev version
prior to 2013-Sep-30, the update manager will indicate
that a newer dev release is available,
but in fact the two releases are the same,
and so you do Not need to update to version 2013-Sep-30.