The only thing I've used Schema for is for easily seeing the differences between what's in the db and what Drupal expects to be there. Very handy for cleaning up a dirty db :) This module can also be used to do all sorts of other db related stuff and has an API used by other modules.

Replaces administrative overview/listing pages with Views, including Views Bulk Operations.

This module should not be enabled in production environments. These views should be used as starting points or examples only and should be exported and used in custom modules if they are to be used in a production environment.

Easily create a scaffolding for a module. Gives a nice UI with a full list of hooks to be included, all organized into collapsable sections. Try it out. It might save you some time.

"It allows a user with the right permissions to switch users."

This module shows if Drupal or any of its modules have been changed. If you have the Diff module installed, it will show you the exact lines changed.

This is the module that is used on this site for code highlighting.

This module integrates with so many other modules but out of the box can show you detailed node revision diffs.

Firebug for Drupal themeing. See the screenshot and screencast, http://drupal.org/node/209561 . Note that this module injects markers into the DOM to do its magic. This may cause some themes to behave erratically and less capable browsers may make it worse (especially IE)/. Enable it when needed, and disable it afterwards.