Pathologic

"Pathologic is an input filter which can correct paths in links and images in your Drupal content in situations which would otherwise cause them to “break;” for example, if the URL of the site changes, or the content was moved to a different server. Pathologic can also solve the problem of missing images and broken links in your site’s RSS feeds."

Label Trash

A nice way to add placeholders to input fields. "This module auto-converts form input fields with space-hogging labels into compact mobile-friendly input fields, with placeholders that animate into much smaller labels the moment text is keyed in. "

Context

"Associate menus, views, blocks, etc. with different contexts to structure your site."

This lets you define sections of your sites. Each section is called a context. Each context can be defined in many ways. A context may combine anything from a content type being viewed, a view being shown, a certain role is viewing the site, and much more. You can then do all sorts of things based on the context; things like theme switching, enabling

Theme developer

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.

Extended File Field

The gist: "This module extends the functionality of the core 'file' field, adding new capabilities to the file upload widget, and providing a new extendible field formatter for displaying lists of files."