There are many collage and poster apps, but I am really enjoying Moldiv. This app is extremely easy to use. You just tap on a frame (similar to a template) to select your photo layout and then tap on a section to add a photo. You can use the photos from your photo gallery or take a new one using the camera. Once you add in your photos, you can tweak the layout by adjusting the frame width and adding a background fills. For the creative amongst us you can add photo effects and decoration features such as stamps.
Moldiv also allows you to add text. Text can also be manipulated to different fonts, effects and colours.
This is a must app and best of all, it is free.