iOS: Instead of paying for Apple's iOS version of iMovie, Splice is an alternative video editor that you can use for free. It comes with many of the same features you'll find in iMovie, but without the cost.
Splice lets you cut, clip, and edit your videos on your iPhone or iOS, much like iMovie but without the cost. You can add multiple video clips back to back, choose transitions, and select background music that comes with Splice (two tracks come free, but you can purchase more from within the app) or add something from your iPod's music library.
If you don't like ads while you're editing, Splice can be purchased ad-free for $2.49 — still quite a bit cheaper than iMovie's $5.99 price tag.