The ACL module, short for Access Control Lists, is an API for other modules to create lists of users and give them access to nodes. It has no UI of its own and will not do anything by itself; install this module only if some other module tells you to.
We're aware of the following modules using ACL (let us know if you know of others):
Content Access (optionally uses ACL to provide by-user access control)
The Autosave module automatically saves a snapshot of your content type form using AJAX. If the user's browser or machine dies while editing a node; the edits will be presented to the user the next time they return to the node. The user may toggle back and forth between the last saved version and the version with the edits that were lost and select which of these they would like to continue with.
How often a form is autosaved as well as which content types to enable autosave for are admin configurable.
This module creates a menu bar that is displayed at the top of every page. The menu bar is fixed in place and uses jQuery to position the bar, along with handling menu effects and adding IE compatibility.
Very useful for themes that don't have a good place to put the navigation menu (although it can be used with any menu.)