Nokia Video Trimmer is able to edit any user captured video that is visible in the Photos+Videos hub (i.e. captured with the standard camera app). When you open the app you are presented with a list of recently recorded videos. Tapping on the video will open the trimming editor, with a scrubber control making it easy to set the start and end point of the video. Videos saved in the Video Trimmer app are stored new videos to the Photos+Videos hub (i.e. does not over write source file).
Windows Phone Store description:
This app makes shortening your videos quick and easy, and works for all videos shot on Nokia Lumia phones with Windows Phone 8. Simply drag the slider to the start and end points then use the step controls to adjust the selection to the second. When you're done, save the video to your phone or upload and share it over WiFi or a mobile data connection, but be sure to check your network operator’s data charges first.
Nokia Video Trimmer is a free download from the Windows Phone Store.