You can just start writing JS on an existing page to output components rendered by React. Fair warning though, React is focused on rendering and initializing,

You can just start writing JS on an existing page to output components rendered by React. Fair warning though, React is focused on rendering and initializing, so it doesn't come with a bunch of handy JS functions like YUI or jQuery. But you can use React & other JS libs together, as long as you're careful that the other JS lib doesn't change the markup of a React component (which would take it out of sync.)