Available role: A module for Drupal to allow users to choose which roles permitted to them they currently use
When developing a site it is well known you need to log in as different users to properly test functionality, or you will always be surprised by incorrectly-configured permissions.
However, it should be much easier to test out working as a given user role by anyone with these privileges.
Take drupal.org or groups.drupal.org itself: once one has special privileges there (as i have been granted this year to help out with giving project promotion capability and to make aliases for groups), it is hard to know what a normal user can or cannot do. Your most trusted users should not have to create a fake account to do things and experience the process as a non-privileged account (documenting as they learn).
* Allowing the default mode to be the least permissive can prevent accidental damage (you can only delete a vocabulary if you power up to do so, for instance).
* Showing only an 80% use case role's base operations by default gives a cleaner user interface.
Kathleen Murtagh set up Masquerade as a sort of sudo system— you use the site as a normal user most of the time, and when you need more privileges you masquerade as an all-powerful sudo user (should probably have called this root, but anyway...). This proved the viability of the concept but using Masquerade to change users is not optimal.
The part of this i care about most seems to exist as the Sudo module for Drupal 6.
It looks like it needs to be expanded slightly as far as permitting a default base roles and then allowing the user to choose any role granted to them. Possibly Permit Own Permissions could be a home for this functionality if the Sudo maintainer is not interested, but Sudo looks like the best fit.