I've used picnik before and thought it was really good. My opinion hasn't changed.
Pros
Auto fix really good.
Has a Straighten feature in Rotate.
When resizing you can keep the proportion.
A lot of pretty advanced features to be free.
Not a community
Really Really easy to get your picture saved to your computer.
Very user friendly, easy to use.
Cons
Can only upload one picture at a time.
No auto syncing.
Some features are only available if you upgrade for $24.95 a year (this could be a pro, but we're dealing with free picture editors.)
Basically I advise everyone get account even if you have Ps.
Below are the pictures I used to test.