Scut is a wonderful library to make Sass even better. There are a few mixins that I find particularly helpful and use quite frequently.

General

Hide an element visually – scut-hide-visually

Create a triangle with the border trick – scut-triangle

Layout

The ubiquitous clearfix – scut-clearfix

Easy margin where you want them – scut-margin

Just pad what you want – scut-padding

Absolutely minimal cruft – scut-absolute

Typography

@font-face with all the “fixins” – scut-font-face

Usage

I almost always use the @include form rather than the @extend form so that I can easily use the styles in media queries.