There are several ways to create a collage with your digital pictures: doing it yourself from scratch with Photoshop (which obviously requires a certain Photoshop knowledge), using an automated Photoshop action that does almost all the hard work for you or simply turning to easier, more specific collage-oriented tools like Shape Collage.
This awesome program lets you create completely customizable photo collages in just a few minutes, and without any artistic or technical skill required. As long as you know how to select photos from your hard drive, and tweak a few simple configuration settings, you're good to go. Shape Collage is completely free for personal use, and works on Windows, Mac and Linux.
But if you're the kind of person who prefers not to install software if there's a web app that does the same job, we've got good news for you: Shape Collage has just launched an online version! Shape Collage Online is a stripped-down version of the desktop app, with less options and reduced functionality, but it works perfectly fine for the occasional photo collage. Just select the photos you want to use in your collage (they must be all online, either as standalone photos or photo galleries), pick a shape for your collage and hit 'Create'. After a few seconds, your collage will be ready!