Ezthumb is a powerful open source program to generate thumbnails (i.e. contact sheets, screenshots or previews) from videos.
It uses powerful libraries from ffmpeg, therefore supporting the most various video codecs. Here is some of the popular video formats Ezthumb supports: AVI, FLV, OGG, MOV, MKV, MP4 (H.264/x264, HEVC/H.265/x265), WebM, WMV. (For a complete list, visit this link: https://ffmpeg.org/general.html#Video-Codecs)
Its interface is based on IUP (http://webserver2.tecgraf.puc-rio.br/iup/), a multi-platform toolkit for building graphical user interfaces, so it looks nice and even on both GNU/Linux and Windows.
It supports 4 image output formats to generate thumbnails: GIF (supports transparency), Animated GIF, JPEG and PNG (supports transparency).