Do you like the retro/old-school filters like those used by Instagram? You can use Photoshop actions to replicate similar effects to your photos. If you’re like me and don’t want to spend the money on Photoshop, give Picnik a try. There are other options to Instagram you might want to take a look at.
Several effects are available on Picnik. I like to use the Vibrance effect when colors are flat and I’d like a bit more contrast.
I’ve always been a fan of Picasa. I only wish that Google incorporates Picnik’s filters into Picasa’s desktop application. Editing photos online can be a bit slow. I doubt that Google will add Picnik effects to Picasa since they’re pushing to do everything in the cloud, but one can always hope.