In case you don't understand what I'm talking about, here's an example:
Pic-Frame Manager is a useful iPhone app that lets you create those photo collages.
Using Picframe is pretty straightforward, you start off by choosing what frame you want, could be two pictures next to each other, one big picture on top and two smaller on the bottom, etc..., there are a bunch of templates to choose from. Then you pick what photos to put in each frame in your collage, and finally play around with the different styles.
You can choose colors and textures for the frame itself, change the sizes of each frame, and set the corners of the frames to be round.
Overall, it's very easy to create your picture frame and then you can either save it to your camera roll, send it by email, or share it on social networks like Facebook and Twitter.
Pic-Frame Manager costs $0.99 and there is an iPad version that also costs $0.99.