Steply Not just photo sharing. Put photos, location, and activities together, add points system for rewards. Mix well. Free.
Foodspotting Share what you're eating, or find great eats by browsing visually in this location based photo app. Free!
Hipstamatic Buy it for $1.99 and get the standard frames and effects, or add on more effects and frames with a Hipsta-pack for $0.99! This one is fun, but not free.
Instagram Like Hipstamatic, you can apply different filters to your photos with Instagram, but unlike Hipstamatic, Instagram is free!
Darkroom This is a nice little app you can try to decide whether you want the pro version. Allows sharing photos. Free.
Flickr If you have a Flickr account, you can look at your photos and and add new ones with this app. Flickr is photo sharing. Free.
Adobe Photoshop Express Nice free app. Limited photo editing abilities, but the basics that are frequently used are there. I like having this one around to smooth out closeups and give them a softer look.