If you - like me - are a fan of keeping your stylesheets readable with Sass and structuring them following the guidelines from SMACSS, you should totally know about this little syntax gem:
color: red; }
SMACSS suggests that we favor single class-name selectors over the nested child selectors that are oh so easy to create using Sass. Using this ampersand-syntax we get selectors that are almost as easy to write - and especially to maintain.
A bit about Rails compatibility
This was added to Sass at version 3.3.0. Unfortunately the current version of sass-rails depends on Sass 3.2.x, so you won't get this out of the box on Rails.