Free open source tools and resources have been getting online over these past few years. Many of these tools are geared directly towards responsive web design, handling CSS and other typical features. The performance of these tools can vary greatly which is why so many designers are hesitant to jump right into a new library.
However I think there are enough in-browser tools and open source libraries to host this amazing showcase. I have put together 25 outstanding responsive design tools for web developers all around the world. Many of these are self-hosted on webpages for testing your responsive layouts and building grids on-the-fly. But I would also recommend the collection of jQuery plugins and alternate CSS libraries which may come in handy down the road.