ComicSketch

http://www.mainada.net/comics/
A tool for people who can actually draw! Create all content via pen and color tools.

ToonDoo

http://www.toondoo.com/
Create your own 1, 2, or 3 panel cartoons using a library of cartoons, text bubbles. Images can be uploaded and manipulated with simple tools

Kerpoof

http://www.kerpoof.com/
Teacher site - http://www.kerpoof.com/teach
Kerpoof is a fun to use cartoon tools with stock shapes and character bubbles, but can also be used to create movies.

Bubblr

http://www.pimpampum.net/bubblr/
Pick flickr photos by tag or user name and put on a timeline, overlay bubbles with text.

Pixton

http://www.pixton.com/uk/
Create simple or complex comics using characters, scenes, objects. All objects are highly editable, and characters can be moved into almost any pose.
Costs $$ to use but is reasonably priced.