I wanted to play around some photographs earlier today, but was away from home, and thus away from my "fancy" photo editing software. Rather than put it off until I got back to my own computer, I decided I'd give Picnik a whirl (using the Flickr interface). I've recommended it to others as a great product photograph editing tool, but hadn't tried it out for artsy shots.
It was fun, there were plenty of free effects, though it looks like some really useful tools like cloning are only available with a premium account. If you're interested in sprucing up some of your images, but aren't ready to use more complicated software, I'd definitely recommend playing around with Picnik.