App Store link: 
I must confess I’m addicted to I Can Haz Cheeseburger. This lets you access Zoomr or the Flickr Can Has Cheeseburger group to see the kittehs. You can thumb through them. The downside? You get whatever photos get added to the public group. I saw a guitar once. No animal, no caption, just the fretboard of the guitar. And occasionally you’ll run across a LOLpolitician.