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.