for one. There are more, way more. But this one is quite safe, or as safe as green stuff is going to be for carnivores.
And then there is "cat grass". That seems to be a combination of grasses, mainly green and tender new growth.
Why do cats vomit? And why do they eat plants?
I have no better answers then the wikipedia article on cat & nutrition. Let me quote the relevant part:
Cats do eat grass occasionally. A proposed explanation is that cats use grass as a source of folic acid. Another is that it is used to supply dietary fiber, helping the cat defecate more easily and expel parasites and other harmful material through feces and vomit.