Graph.tk is an online and open-source graphing utility.It's very easy to use. Use the "+" button to create a new equation. Use "/" for fractions, "^" for exponentials, and "*" for multiplication. No install necessary. Runs straight from within the browser using the new HTML5 features. But it can still save all the graphs, and it still works when you're not even connected to the net! You will just need to have a good modern browser (Firefox 4+, Google Chrome 14+, Internet Explorer 9+, Opera 11.60+, Safari 5+).