You have a couple different options if you want a free flowchart generator. Below is a free desktop software that runs on mac, windows, and linux.
http://www.yworks.com/en/products_yed_about.html
Another option would be to use a free web app such as this one:
http://www.slickplan.com/
There are a couple benefits of using the web app as you will be able to use the app on any computer without downloading and installing any software. You don't have to worry about upgrading the app as that is done automatically.
Desktop software has it's own benefits as it may have more features and be more capable of more advanced things that it can do.
Both yEd and slickplan are free, so I suggest you try both of them and see which one you like better and fits what you need to use if for.
