Picnik

You probably have hundreds or thousands of digital photos on your PC. And a lot of those photos would probably benefit from a little tweaking. But that doesn't mean that you have to download and install photo editing software. Picnik supplies a nice suite of tools for editing photos online. All you have to do is upload your photos, or have Picnik grab them from a site like Flickr (which doesn't have editing features), and then get to work. Picnik offers tools aplenty for performing simple editing--cleaning up red-eye or resizing photos, say--as well as doing more-extensive work, such as changing the exposure, fixing a color cast, or applying special effects.

You will find tons of tons of special effects, from artsy to fun. There is no download required, nothing to install! And most of all... it works on Mac, Windows and even Linux

They offer both the free version and a premium version.