I use Camera Plus Pro. it has amazing set of photo filters (more than 40 to choose from). it can even record videos with video filters. the best part is i can upload the pics after adding effects on it directly to facebook.
Just search for Photo Effects. There are tons of different apps out there, and some are even free (although the better ones usually have a cost). Hope this helps.
Hisptamatic is a good photography app. It has a lot of colorful filters. You can make a lot of artistic photo effects. It's used by "iphoneographers". A new breed of photography. If you're interested in looking at the pics go to Flicker under "iphonography".
Filterstorm is a great photoapp. It can do pretty much anything imaginable to a photo! Although it's $3.99 in the app store, I think it's well worth the price!