Everyone knows about Twitter Bootstrap. It's a great framework, but it's not your only option. Here are some other wonderful frameworks to choose from.

1. Zurb Foundation

Foundation is the first front-end framework created for developing fully responsive web applications. If you're a new user and looking to learn the framework, they've made it a lot easier with a full suite of support options. They have building blocks, a dedicated forum, videos, and web based/on site training classes.
Get Foundation

2. Skeleton

If your making a simple website, with only a few pages, and looking to get started fast. Skeleton is your best bet. Skeleton has utility classes which make it much easier to do things like float elements.
Get Skeleton

3. Centurion

Centurion, is very interesting. It's scalable and easy to use syntax let's you web prototype very quickly and it offers the latest in CSS3 and HTML5.
Get Centurion

4. Groundwork

Groundwork offers a great grid system over which you can build a layout that will fit on any screen size.
Get Groundwork

5. Metro UI

If you like the Window's 8 style, choose Metro UI. It has all the common components you need delivered in Windows 8 styled interface.
Get Metro UI