Quick Look support for non-native video files (.avi, .mkv) on Mavericks
I've uploaded QuickLook and Spotlight plugins here: https://github.com/Marginal/QLVideo/wiki . These allow Finder to display thumbnails, static previews, cover art and metadata for most types of video files, including .asf, .avi, .flv, .mkv, .rm, .webm, .wmf etc.