Comic Creator invites students to create their own comic strip. Characters and props and backgrounds are a bit limited, but not a bad start. They also have resource lessons for various grade levels from K-12.
Completed cartoon cannot be saved, but can be "printed" either on actual paper or as a PDF document. [With Macintosh this is a standard "print as pdf" operation; with Windows, I believe you need a print as pdf plugin. There are several free ones out there.]