a ravioli stamp. Not that expensive and works nicely...

A round medium glass.

I make squares with a pizza wheel.

Cut a small can in half. Done.

I always just make my ravioli without any tools (because I don't own any!). I roll out the pasta dough by hand into long strips (that are a little more than twice as wide as I'd like my finished ravioli to be), dollop tablespoons of filling on one side, about two inches apart, fold over and seal with water (making sure to get all the air pockets out), and cut and trim them into individual ravioli with a sharp knife. I think it might be quicker and easier to use stamps and pasta rollers, so I'm thinking of investing in some, but my way works pretty well, since I don't make ravioli on a regular basis. Good luck with your own ravioli making!