Material Design Lite
I would recommend against using it at this stage, for a couple of reasons:
It is much less complete than you might expect from a CSS framework. There is little support for forms elements, for example.
Markup is somewhat akward.
It does not play nicely with livereload and will constantly scroll the page to the top.
Some other Material Design CSS + JS Frameworks with partial pending release:
Material Design for Bootstrap (note that this is incomplete, and free only for non-commercial projects)