Avidemux is a free video editor designed for simple cutting, filtering and encoding tasks. It supports many file types, including AVI, DVD compatible MPEG files, MP4 and ASF, using a variety of codecs. Tasks can be automated using projects, job queue and powerful scripting capabilities.
While not as powerful or filled with features like Sony Vegas or Adobe Premier, Avidemux features a simple and intuitive user interface that is easy to understand. Plus Avidemux is fully open source and available in Windows, Mac and Linux versions. Head on to the official project page for the latest version.