While we're plugging our favourite Bootstrap alternatives, I just want to give a shoutout to Bourbon: http://bourbon.io/
I find it a lot more flexible than Bootstrap, but equally useful for fundamentals and cross-browser support. It is also less opinionated about the HTML markup and styling, at the cost of requiring more setup (rather than just, say applying classes) than Bootstrap. But with the additional modules of Neat, Bitters, and Refills, you can get the same sort of UI suite going quite easily.
This is apples vs. hammer comparison. Totally different tools for different purposes.
Bootstrap is for copy-paste website/app sketching and bourbon is for speeding up SASS coding. I tried it, but I ended up using 1-2 functions of it, so it got dropped.
I find it a lot more flexible than Bootstrap, but equally useful for fundamentals and cross-browser support. It is also less opinionated about the HTML markup and styling, at the cost of requiring more setup (rather than just, say applying classes) than Bootstrap. But with the additional modules of Neat, Bitters, and Refills, you can get the same sort of UI suite going quite easily.