If you want to make giant posters, for say.... an OccupyWallStreet protest. Posterazor is an awesome little application. It's open source, easy to use and works on Windows, OSX, and Linux.
Also Rasterbator has a pretty great online wizard and the output looks much nicer (depending on your source image) than Posterazor but it has a 1MB upload limit. There is also a Windows version of Rasterbator, but nothing for Mac or Linux.