A couple of years ago I had wanted to do something fun for my roommate's birthday. I had pondered filling the room with balloons, arranging a surprise party, and a few other ideas. Eventually I decided it would be great to get him a lifesize poster of his favorite actor. Now being short on funds and time, I couldn't get my hands on a professional quality print. Luckily I had a plan.
I remembered a neat little tool called "The Rasterbator" that Homosaku puts out. The tool uses a process called rasterization and will take an image, expand the pixels, and give you a multi sheet document to print out. Once this document has been printed, clipped, and assembled, you have yourself a large banner of your original image. This tool is great for decorating your living space on the cheap, making banners for parties, and a variety of other situations. There's even a gallery where you can see innovative uses of the tool both in content and the placement of the banners.
The online tool has some file size restrictions, but you can also download a version of the program to your computer that doesn't have any of the restrictions.