Posterini is a website that allows users to upload a picture and then create a downloadable and/or purchasable poster. There are several features that allows users to edit the poster and the text on the poster to meet their needs. It uses a simple interface, but simple enough to create a product.
English teachers could use this to turn a book into a poster for a movie. Social Studies Teachers could use it to create a poster advertising a historical event. There are several ways that a poster could be created for a classroom project.
What is great about this simple product is that it does not require any registration to use, just the ability to select and upload a photo. I recommend using it first and playing around to see what all it can do and then devise a plan for integration into the classroom.