Windows: Microsoft's Movie Maker is already the best free option available for movie editing on the Windows platform. The newly-released 2012 version adds some handy new features, including tools to locate music for your videos rather than skating around copyright laws and diving into your iTunes collection.
Within Movie Maker, you can choose to search for music from AudioMicro, the Free Music Archive and the Vimeo Music Store. Other enhancements include the ability to outline captions so they don't blend into your video, switching to H.264 as the default save format, and adding an option to upload direct to Vimeo. Windows 8 users get an automatic stabilisation tool for shaky videos.
Movie Maker 2012 is a free download for Windows. It's part of the Windows Live bundle (and easily the most useful app in that software set).