If you're looking for light-weight Sass frameworks, you might like Thoughtbot's mixins and libraries.<p>- Bourbon (bourbon.io, like Compass but lighter)<p>- Neat (neat.bourbon.io, grid framework via mixins)<p>- Bitters (bitters.bourbon.io, scaffold styles and file structure)<p>- Refills (refills.bourbon.io, example components built with all of the above)<p>I've used these in multiple projects and I wouldn't switch back to Bootstrap or Foundation, which end up adding tons of unused styles to projects, and IMO, get in the way. The advantage to Bourbon, etc... is that you have much more control over the structure of your Sass and what specifically you need. If you're looking for out of the box UI, Refills gives you something to work with, but honestly, I think it's tailored towards developers that are building custom interfaces.
Bootstrap and Zurb foundation are probably the most popular these days.<p>If you're looking for something super light weight and responsive you might also want to check out getSkeleton.com<p>PureCss.io is also nice if you're looking for modular options.
I like simple grid. Just a simple grid system as the name suggests. Less than 1kb minified.<p><a href="http://thisisdallas.github.com/Simple-Grid/" rel="nofollow">http://thisisdallas.github.com/Simple-Grid/</a>
UIKit (<a href="http://getuikit.com/" rel="nofollow">http://getuikit.com/</a>) is my personal favourite. Pure (<a href="http://purecss.io/" rel="nofollow">http://purecss.io/</a>) is nice too.