From http://thesassway.com/beginner/how-to-structure-a-sass-project

sass_project_layout.txt

stylesheets/
|
|-- modules/              # Common modules
|   |-- _all.scss         # Include to get all modules
|   |-- _utility.scss     # Module name
|   |-- _colors.scss      # Etc...
|   ...
|
|-- partials/             # Partials
|   |-- _base.sass        # imports for all mixins + global project variables
|   |-- _buttons.scss     # buttons
|   |-- _figures.scss     # figures
|   |-- _grids.scss       # grids
|   |-- _typography.scss  # typography
|   |-- _reset.scss       # reset
|   ...
|
|-- vendor/               # CSS or Sass from other projects
|   |-- _colorpicker.scss
|   |-- _jquery.ui.core.scss
|   ...
|
`-- main.scss            # primary Sass file

• The modules directory is reserved for Sass code that doesn't cause Sass to actually output CSS. Things like mixin declarations, functions, and variables.
• The partials directory is where the meat of my CSS is constructed. A lot of folks like to break their stylesheets into header, content, sidebar, and footer components (and a few others). As I'm more of a SMACSS guy myself, I like to break things down into much finer categories (typography, buttons, textboxes, selectboxes, etc…).
• The vendor directory is for third-party CSS. This is handy when using prepackaged components developed by other people (or for your own components that are maintained in another project). jQuery UI and a color picker are examples of CSS that you might want to place in the vendor directory. As a general rule I make it a point not to modify files in my vendor directory. If I need to make modifications I add those after the vendored files are included in my primary stylesheet. This should make it easy for me to update my third-party stylesheets to more current versions in the future.