Easy Poster Printer lets you make posters at home. You don't need a special printer. This program blows up any image. Then, it cuts the image into page-sized chunks. You print them out and piece the poster together. It's pretty simple.
http://www.gdsoftware.dk/products.aspx
Thanks, Chris. Tried it out and it works. Very simple to use - even I could manage it. Excellent.