Recently discovered this very useful app called CollageItFree. It's a drag and drop photo collage maker for both windows and OS X. Here's an example of what it puts together. You select the photos, make a few preference selection like number of photos, border treatment and size as well as background.
There is a free version as well as pro (pro cost is $19.90). Why it appealed to me — it has a lot of flexibility and takes no time to put a collage together.
