InstaPicFrame (free): Used for making collages/adding multiple photos to a single IG post by adding a frame.
InstaWeather (free): Used for adding a cool weather or forecast stamp onto your photo. I love doing this especially if I'm on vacation, because it's a fun photo to add into the album to show what weather was like each day, and I'll be using it again during AWITL, I think. I also have a lot of friends and family around the world, so it's fun for each of us to see the other's weather easily.
InstaPlace (free): I probably use this one least of all, but it's worth a mention. It adds a fun stamp like "Greetings from ________." Again, I use it a lot when I travel for sending out postcards or for adding into photo books.
Postagram (free app, postcards cost $0.99): I love this app so much. Seriously. For $0.99 per postcard (additional for international, but YES they do global shipping, or less per postcard if you buy credit packages in bulk), you can send a postcard of any of your instagram pics to anyone! I love to send little postcards of my son to my grandmother, since she lives 3 hours away. This year, after my son's birthday, I sent postcard thank yous to out of town relatives who sent gift cards or money, with a short thank you and a description of how he spent the money (or if he saved it). My mom used it to send a picture of her new students on their first day of school to the main office (she works for a foreign exchange company). It's a really fun app that allows for a lot of creativity. I even sent out my postcards from Vacation using the app-- InstaPlace with an island landmark for anyone who didn't go with us, and then (since it was an extended family vacation) snapshots I took of my the various people who went sent to them with a message of how much I loved hanging out with them. Best part? The message is printed on the postcard, but also on the back of the picture, and the picture easily pops out so they can just save that photo with the message, instead of the whole card.
Walgreens Mobile App (free, 4x4 prints $0.39 each and 8x8 prints $8.99 each): Another app I use all the time. I can order prints from my phone and they're almost always ready within half an hour for me to pick up. They'll pull my photos directly off of Instagram and then allow me to pick how many I want to print and choose my size. The quality is actually pretty good for it being an IG print, and I'm a pretty big fan of the speed. It works really well for PL and other times when you need a pic "now."
Anyway, those are my go-to photoelated apps.