Shine Video Cutter is a nice video edition tool for the Mac. It can be used to trim your video files into shorter clips. Most video converters come with this feature built in, but if you need to make a shorter clip out of a video file, and you don't own a video converter, then you can get this standalone trimmer.
You can add a single file or an entire folder to load videos into the interface. The app has very good input format support. I successfully opened and processed MKV, MPG and AVI files with it and I didn't detect any problems when playing or trimming those videos.
The trimming process is very simple. You can start playing the video using the built-in video player, and by clicking the "B" and "E" buttons, you set beginning and end times. The video between those times will be saved when you trim it. You can also use the video converter to change the format of this new video to any of the support ones. The list is extensive and most popular formats are supported. There are even conversion profiles for popular multimedia playback devices.