The yEd Graph Editor application is completely free to use software, both for private as well as commercial use (subject to license terms).
If you find yEd to be a most valuable tool, a real gem, and want to "pay back" by making a donation, for example, we very much thank you for the offer, however, we do not accept donations.
As an alternative, though, if you recommended yEd to your friends, we surely wouldn't have any objections. :-)
And if you are blogging about yEd, which is also very much appreciated, let us know via Trackback!
Trackback URL: <http://www.yworks.com/trackback/yed>
P.S.: On a related note, you can also support us and help us to improve yEd by giving feedback. Especially when you encounter a problem while using yEd, we would appreciate your short description that helps us to reproduce the issue.