Have you heard of Stixy.com? I recently had the privilege of attending FETC 2013 and I came home with a plethora of technology resources. I immediately came back to my classroom and implemented several of them. This is one that I couldn't wait to try. Stixy.com is an online collaboration tool.
You have to sign up and create a free account but your users do not. You can password protect it. I made a simple password that my students will use for every Stixy that we create.
When setting it up you can add pictures or prompts. This one I used this week for my Bible lesson. Students go to the unique web address for the Stixy you created. They can then login with the password. The students add sticky notes and type whatever you tell them.
In my Stixy above, I used it as review. Students worked in groups to write what they remembered about the previous day's lesson on Nehemiah.
This is a website not an app, but it does work on Safari on the iPad. You could assign this as homework and let students add stickies from home if you don't have multiple computers in your classroom. You could also make it a center and students could enter their responses on your classroom computer.
The beauty of this is when you refresh your browser you can see the responses in real time.
Try it, you'll like it!