"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."

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. "

"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

"Internet Explorer implements a proprietary technology called Conditional Comments..."

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."

Seems like a nice way to get a responsive grid for views without doing any custom theming. I haven't used it yet.

Easily get custom fonts on your site.

This shows you all the different themeable elements and lets you see how your theme renders them.