Snapseed This is what I call the baby photoshop - the best photo optimizer by far. It also has some intriguing special effects like grunge, drama, and frames. It's robust so to get the most out of it, tap the question mark in the upper right to get help and links to instructional videos online. I always say, if you can only have one app, this is the one! And it's free!!!! The above photo of palm tree and gazebo roof was optimized in Snapseed and then zapped with the Drama filter in this same app.
Pro HDR HDR stands for HIgh Dynamic Range and this app is a necessity for any iphoneographer. Anytime you have bright highlights and dark shadows (eg anytime you are shooting outdoors on a sunny day) you can get a photo that exposes for both areas, giving you a perfect photo that looks like what you see. Set it on auto, hold it still while it analyzes and shoots 2 photos and then it will combine them into one. Save the result to your camera roll. Then app it with some other apps or just sit back and wait for compliments. The gazebo at sunrise below would have been a silhouette against the rising sun without ProHDR.
TouchRetouch Want to get rid of power lines, annoying poles or trash in your photo? This is the go to app for content aware healing and removal of unwanted objects. Open your photo, use the little paint brush icon to select the part you want removed, and hit the go button (shaped like a little right arrow). Then just like magic, it's all fixed.
Autopainter Cool painting app that is absolutely intuitive. Just use a photo from your camera roll, pick from a number of styles, and voila - amazing photo art. Works so well with plants and architecture. Two variations below with the laua`e fern and the palm tree (lithograph style).
Blender Great app for combining images - very intuitive interface. Just import one and then another photo from your camera roll and it will put them together. It's like a double exposure or collage effect. After you choose two photos, you can also choose from a number of blend modes - go up to the two overlapping squares at the top and it will bring them up. Try combining unlike images for a surreal effect. Terrific creative fun. The photo below was created with blender combining two shots of palm leaves and blend mode exclusion.
Other apps you might want to experiment with:
Simply BW Absolute best B and W app - like a teeny Ansel Adams living in your phone.
Autostitch Panoramic - best. Can shoot panos from within the app or shoot with camera and combine later. It can handle lots of shots and big files.
Perfectly Clear Best 1 stop shop to fix pix with no muss, no fuss, superior noise removal
This will keep a photographer busy for years. And likely whet your app-etite for more.