Not everybody has a plotter at home, so if you want to print large-sized posters in your humble A4 printer, your best option is Posterazor.
This open-source tool splits up any image into several sheets of paper, enabling you to print them out and then build a poster as large as you want it to be. Just bear in mind that you should use high definition photos, because otherwise you'll get a nice pixelated design instead of a huge attractive poster to pin up on your bedroom's walls. The good thing is that Posterazor supports a really wide variety of image formats.
Posterazor works as a wizard, guiding you through five simple steps in order to create your poster. Though this is probably the easiest way to go for novice users, I missed having a somewhat more advanced interface with other extra options such as frames and borders, some guiding lines to cut out the image or the possibility to add a text layer or even apply a watermark, like in Posteriza (why do they all have similar names?).
The final poster is created in PDF format, so you'll need a PDF reader in order to open the file and print it. I personally recommend Foxit Reader but any other will do the trick. And I must say the results were gorgeous!
Despite lacking some advanced features, its ease of use and support for a wide array of image formats make of Posterazor a great tool for printing large posters.