We love all the little things you can do with QuickTime, but The Sweet Setup points to one I've never noticed before: the ability to stitch a bunch of videos into one simple timeline. Just open one video, then select and drag a bunch of videos from Finder on top of the first video. After you drop them, you'll notice you can insert them anywhere, reorganise them and trim select clips. When you're done with your jury-rigged edits, click File > Export to create a single file with your edit. This is extremely useful if you need to just cut together a quick set of videos but don't want to download a whole program just to do it.
How to combine videos in QuickTime on OS X [The Sweet Setup]